Ensure IsVectorPerElementMask does a correct check and use it in more places

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Pull request overview

This PR refines the JIT’s notion of “per-element masks” for SIMD/MASK values and applies that stricter check in additional folding/morphing/value-numbering scenarios, aiming to enable safe optimizations while avoiding mismatched mask element-count issues.

Changes:

• Adds ValueNumStore::GetConstantSimd to centralize SIMD constant extraction and uses it in existing SIMD constant checks.
• Expands/aligns IsVectorPerElementMask logic in both GenTree and ValueNumStore (including support for MASK constants and element-count validation).
• Uses the updated per-element mask checks in more folding/morphing paths for comparisons against Zero / AllBitsSet.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/coreclr/jit/valuenum.h	Adds GetConstantSimd declaration; updates HWIntrinsic VN helper signature; declares IsVectorPerElementMask in VN store.
src/coreclr/jit/valuenum.cpp	Implements GetConstantSimd; adds VN-based IsVectorPerElementMask; updates folding logic to use it; adjusts HWIntrinsic VN helper.
src/coreclr/jit/simd.h	Makes helper array predicates const-correct; adds an overload to check per-element mask-ness for simd_t given base type + element count.
src/coreclr/jit/morph.cpp	Uses IsVectorPerElementMask to guard compare morph optimizations involving per-element masks.
src/coreclr/jit/gentree.cpp	Strengthens GenTree::IsVectorPerElementMask to validate MASK constants and intrinsic compatibility; uses it in HWIntrinsic folding.