compiler: add Enum.reduce_while/3 for integer range compilation

[yevbar]

Summary

Add compilation of Enum.reduce_while/3 over integer ranges to efficient tail-recursive WASM loops with early termination.

Motivation

Enum.reduce_while/3 is a common Elixir pattern for reductions that may terminate early based on a condition. Previously, this function wasn't supported in the compilable subset, forcing users to write manual recursive functions for early-exit accumulation patterns.

Implementation

The reducer function body uses {:halt, val} and {:cont, val} to signal early termination or continuation. These are transformed at the AST level before IR compilation:

• {:halt, val} → return val immediately (loop exit)
• {:cont, val} → recursive call to helper with val as new accumulator (loop continue)

Free variables from the outer function scope (e.g., function parameters like threshold referenced inside the reducer) are automatically detected and threaded through as extra helper function parameters.

Supported patterns

• Plain ranges: Enum.reduce_while(1..n, init, fn i, acc -> ... end)
• Stepped ranges: Enum.reduce_while(0..n//2, init, fn i, acc -> ... end)
• Negative steps: Enum.reduce_while(n..1//-1, init, fn i, acc -> ... end)
• Dynamic steps: Enum.reduce_while(a..b//s, init, fn i, acc -> ... end)
• Block bodies with let bindings
• Nested if/case/cond with halt/cont in any branch
• Closures over outer function parameters

Example

# Sum numbers until the sum exceeds 100
Enum.reduce_while(1..1000, 0, fn i, acc ->
  new_acc = acc + i
  if new_acc > 100, do: {:halt, new_acc}, else: {:cont, new_acc}
end)

Compiles to a tail-recursive helper that TCO converts to a WASM loop with br_table exit.

Tests

11 new tests covering:

• Basic reduce_while with halt conditions
• Range exhaustion (never halts)
• Immediate halt
• Accumulator transformations
• Stepped ranges (positive, negative)
• End-to-end WASM execution with Wasmex
• IR generation verification
• Optimization pass compatibility
• Block bodies with let bindings