vm: compile short-circuit and/or for the dispatcher

[shinyobjectz]

A function containing and or or was never compiled for the dispatcher. The encoder had no lowering for :test_and / :test_or, so it returned :fallback and the whole prototype ran on the instruction interpreter. That is common in real code: x = x or default, return c and a or b, guards in if conditions.

Change

• Two new opcodes, @op_test_and (77) and @op_test_or (78), encode {tag, dest, source, body}. When the left operand decides the result it is copied into dest. Otherwise body (the right operand, ending in a write to dest) runs as a nested block and resumes through the same cont marker :test uses.
• No changes to codegen or the interpreter.

Tests

• test/lua/vm/and_or_engines_test.exs: 14 cases run under both engines. They cover value-returning and/or; nil/false/0/"" operands; nesting; right-operand side effects that must not run; and/or in if, while and repeat conditions; default-argument assignments to locals, fields and upvalues; multi-return truncation; break after an and/or in a loop; a closure built in the right operand; and an error raised in the right operand. Each case also asserts Bytecode.fully_compiled?.
• Some existing tests used and/or to keep a function on the interpreter. max_instructions_test (cross-engine mutual recursion) and peephole_test (:call_self on the interpreter) now strip that function's bytecode instead. The bytecode_test fallback and cascade cases use a synthetic prototype with {:set_list, _, _, 0, _}, the one shape the encoder still rejects. max_registers_invariant_test walks the new opcodes.

The full suite passes: mix test gives 2914 passed, and mix test --include lua53 --include differential --include slow gives 2935 passed.

Benchmark

Each number is the median of 15 :timer.tc runs after 3 warmup runs, in ms. The machine is an Apple M4 with OTP 29 and Elixir 1.20.3, MIX_ENV=prod. "budget" means Lua.new(max_instructions: 1_000_000_000).

program	before	after	before (budget)	after (budget)
fib(25) written with and/or	59.8	34.2	60.0	43.3
date functions (and/or)	11.1	9.3	12.0	10.2
fib(25) with if (control)	29.2	28.9	38.9	38.3

-- fib with and/or
local function fib(n) return n < 2 and n or fib(n - 1) + fib(n - 2) end
return fib(25)

-- date functions: day_of_year for the 15th of every month, 1900-2100
local function is_leap(y) return (y % 4 == 0 and y % 100 ~= 0) or y % 400 == 0 end
local function days_in_month(y, m)
  return m == 2 and (is_leap(y) and 29 or 28) or ((m == 4 or m == 6 or m == 9 or m == 11) and 30 or 31)
end
local function day_of_year(y, m, d)
  local n = d
  for i = 1, m - 1 do n = n + days_in_month(y, i) end
  return n
end
local s = 0
for y = 1900, 2100 do for m = 1, 12 do s = s + day_of_year(y, m, 15) end end
return s