Fix Array2Struct missing handlers for ArrayRMW and ArrayCmpxchg

[sumleo]

Summary

The escape analysis in Heap2Local accepted ArrayRMW and ArrayCmpxchg as FullyConsumes, allowing non-escaping arrays with these operations through to the Array2Struct transformation. However, Array2Struct had no visitArrayRMW or visitArrayCmpxchg handlers. This caused these operations to be left unrewritten after the array-to-struct conversion, and the type rewriting infrastructure replaced them with unreachable blocks — silently turning working code into unconditional traps.

Before fix (array.atomic.rmw.add that should return old value becomes unreachable):

(block ;; (replaces unreachable ArrayRMW we can't emit)
  ...
  (unreachable)
)

Additionally, the escape analysis for ArrayRMW/ArrayCmpxchg was missing the constant-index check that ArrayGet/ArraySet both have, which Array2Struct requires to map array elements to struct fields.

Fix

1. Add !curr->index->is<Const>() checks to the escape analysis for ArrayRMW and ArrayCmpxchg (matching ArrayGet/ArraySet)
2. Add visitArrayRMW and visitArrayCmpxchg to Array2Struct that convert array atomic operations to their struct equivalents, following the pattern of visitArrayGet/visitArraySet

Test plan

• Added test cases to test/lit/passes/heap2local-rmw.wast:
  • $array-rmw-add: array.atomic.rmw.add on array.new_fixed — fully optimized to locals
  • $array-cmpxchg: array.atomic.rmw.cmpxchg on array.new_fixed — fully optimized to locals
  • $array-rmw-nonconstant-index: non-constant index prevents optimization (left as-is)
• All lit tests pass

[kripken]

Thanks, looks great but for one request for testing.

[kripken]

Please add tests for the out-of-bounds case, that this code correctly handles by emitting drops + an unreachable.