[ICE] Const generics + Const normalization + Specialization

Found while toying around [#20400](https://github.com/rust-lang/rust/issues/20400).

### Code

```Rust
#![feature(const_generics)]
#![feature(specialization)]
#![allow(incomplete_features)]

//--------------------------------------------------

trait Depth {
    const C: usize;
}

trait Type {
    type AT: Depth;
}

//--------------------------------------------------

enum Predicate<const B: bool> {}

trait Satisfied {}

impl Satisfied for Predicate<true> {}

//--------------------------------------------------

trait Spec1 {}

impl<T: Type> Spec1 for T where Predicate<{T::AT::C > 0}>: Satisfied {}

trait Spec2: Spec1 {}

impl<T: Type + Spec1> Spec2 for T where Predicate<{T::AT::C > 1}>: Satisfied {}

//--------------------------------------------------

trait Foo {
    fn Bar();
}

impl<T: Spec1> Foo for T {
    default fn Bar() {}
}

impl<T: Spec2> Foo for T {
    fn Bar() {}
}
```


### Meta

`rustc --version --verbose`:
```
rustc 1.45.0-nightly (74e804683 2020-05-30) running on x86_64-unknown-linux-gnu
```

### Error output

```
error: internal compiler error: src/librustc_traits/normalize_erasing_regions.rs:37: could not fully normalize `Const { ty: usize, val: Unevaluated(DefId(0:4 ~ playground[3575]::Depth[0]::C[0]), [<T as Type>::AT], None) }`
```

<details><summary><strong>Backtrace</strong></summary>
<p>

```
thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:490
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_middle::ty::context::GlobalCtxt::enter_local
  21: rustc_traits::normalize_erasing_regions::normalize_generic_arg_after_erasing_regions
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::normalize_generic_arg_after_erasing_regions>::compute
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_middle::ty::fold::TypeFolder>::fold_const
  27: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::subst_and_normalize_erasing_regions
  28: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  29: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  30: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  31: rustc_mir::const_eval::eval_queries::const_eval_raw_provider
  32: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_raw>::compute
  33: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  34: rustc_query_system::query::plumbing::get_query_impl::{{closure}}
  35: rustc_query_system::query::plumbing::get_query_impl
  36: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  37: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  38: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  39: rustc_data_structures::stack::ensure_sufficient_stack
  40: rustc_query_system::query::plumbing::get_query_impl
  41: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  42: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  43: rustc_infer::infer::InferCtxt::const_eval_resolve
  44: <rustc_trait_selection::traits::fulfill::FulfillProcessor as rustc_data_structures::obligation_forest::ObligationProcessor>::process_obligation::{{closure}}
  45: rustc_data_structures::obligation_forest::ObligationForest<O>::process_obligations
  46: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  47: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_all_or_error
  48: rustc_infer::infer::InferCtxt::save_and_restore_in_snapshot_flag
  49: rustc_trait_selection::traits::specialize::fulfill_implication
  50: rustc_middle::ty::context::GlobalCtxt::enter_local
  51: rustc_trait_selection::traits::specialize::specializes
  52: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::specializes>::compute
  53: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  54: rustc_data_structures::stack::ensure_sufficient_stack
  55: rustc_query_system::query::plumbing::get_query_impl
  56: rustc_middle::ty::context::GlobalCtxt::enter_local
  57: rustc_trait_selection::traits::coherence::overlapping_impls
  58: <rustc_middle::traits::specialization_graph::Children as rustc_trait_selection::traits::specialize::specialization_graph::ChildrenExt>::insert
  59: <rustc_middle::traits::specialization_graph::Graph as rustc_trait_selection::traits::specialize::specialization_graph::GraphExt>::insert
  60: rustc_trait_selection::traits::specialize::specialization_graph_provider
  61: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::specialization_graph_of>::compute
  62: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  63: rustc_data_structures::stack::ensure_sufficient_stack
  64: rustc_query_system::query::plumbing::get_query_impl
  65: rustc_query_system::query::plumbing::ensure_query_impl
  66: rustc_typeck::coherence::coherent_trait
  67: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::coherent_trait>::compute
  68: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  69: rustc_data_structures::stack::ensure_sufficient_stack
  70: rustc_query_system::query::plumbing::get_query_impl
  71: rustc_query_system::query::plumbing::ensure_query_impl
  72: rustc_typeck::coherence::check_coherence
  73: rustc_typeck::check_crate
  74: rustc_interface::passes::analysis
  75: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  76: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  77: rustc_query_system::query::plumbing::get_query_impl
  78: rustc_middle::ty::context::tls::enter_global
  79: rustc_interface::interface::run_compiler_in_existing_thread_pool
  80: rustc_ast::attr::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.45.0-nightly (74e804683 2020-05-30) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<<T as Type>::AT as Depth>::C`
#1 [const_eval_raw] const-evaluating `<T as Spec2>::{{constant}}#0`
#2 [const_eval_validated] const-evaluating + checking `<T as Spec2>::{{constant}}#0`
#3 [specializes] computing whether impls specialize one another
#4 [specialization_graph_of] building specialization graph of trait `Foo`
#5 [coherent_trait] coherence checking all impls of trait `Foo`
#6 [analysis] running analysis passes on this crate
end of query stack
```

</p>
</details>



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[ICE] Const generics + Const normalization + Specialization #72845

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[ICE] Const generics + Const normalization + Specialization #72845

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