Clarify WASM shuffle/swizzle codegen comments

[tannergooding]

Post-merge follow-up to #130850, addressing three review comments from @adamperlin.

• hwintrinsiccodegenwasm.cpp: reword the contained-mask Swizzle->i8x16.shuffle remark. On Wasm genConsumeMultiOpOperands only updates liveness and doesn't touch the value stack, so the comment now says "prior codegen left the source on the value stack once" rather than attributing it to genConsumeMultiOpOperands.

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• gentree.cpp (TARGET_WASM Shuffle-lowering branch): add a lead-in comment describing the element->byte-index expansion -- each element-granular selector expands into elementSize consecutive byte indices for i8x16.swizzle, and an out-of-range selector becomes 0xFF bytes so swizzle's native index >= 16 -> 0 behavior zero-fills that element. Dropped the now-redundant inline else-block comment.

The third comment (lowerwasm.cpp, feeding a constant-zero second operand to i8x16.shuffle instead of the source twice) is addressed in the thread rather than in code: a local.get of the existing source is smaller than materializing a 16-byte v128.const zero, and it can't remove the special case since the Swizzle node is 2-operand (source, mask->immediate) with no second-vector slot.

Comment-only; no functional change.

Note

This PR description was drafted by Copilot on behalf of @tannergooding.

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[Copilot]

Pull request overview

This PR updates explanatory comments in the WASM SIMD shuffle/swizzle code paths in the JIT, clarifying how masks map to i8x16.shuffle / i8x16.swizzle and removing redundant commentary.

Changes:

• Reword the Swizzle-contained-mask comment in genHWIntrinsic (WASM codegen) to avoid attributing behavior to a specific helper.
• Add a lead-in comment in gtNewSimdShuffleNode (WASM lowering branch) describing element-to-byte-index expansion for i8x16.swizzle, and remove a now-redundant inline comment.

Show a summary per file

File	Description
src/coreclr/jit/hwintrinsiccodegenwasm.cpp	Comment clarification for WASM Swizzle lowering to i8x16.shuffle.
src/coreclr/jit/gentree.cpp	Adds/cleans up comments documenting WASM shuffle-to-swizzle mask expansion semantics.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1

[adamperlin]

/ba-g infra failures