[release/11.0] Preserve mask element granularity in SIMD transformations

[github-actions]

Backport of #133708 to release/11.0

/cc @tannergooding

Customer Impact

• Customer reported
• Found internally

This fixes incorrect SIMD results and JIT assertion failures in #133548 and #133552. Both issues reproduce on .NET 11 RC2 but not .NET 8, 9, or 10, so .NET 11 would otherwise ship with regressions in SIMD mask folding and lowering. Because the fixes share the same mask-granularity invariants, this backport also includes #133555, which affects .NET 9 through .NET 11.

Regression

• Yes
• No

The #133548 and #133552 failures were introduced during the .NET 11 development cycle; the exact introducing change was not identified. #133555 predates .NET 11 and is included because it is fixed by the same change.

Testing

The source PR added focused regressions for all three xarch failures and SVE coverage for the shared folding guard. The xarch cases failed with the original JIT and passed after the fix in Checked and Release builds, including native AVX-512 execution and reduced-ISA configurations. SVE before/after cross-codegen preserved the required vector operations. Representative valid same-width optimizations retained identical instruction counts and code sizes.

Risk

Low. The changes are narrowly scoped element-granularity guards and invariant corrections that preserve existing valid SIMD transformations. Focused regression coverage exercises the affected vector widths and ISA configurations, SVE cross-codegen validates the shared guard, and representative unaffected optimizations retained identical instruction counts and code sizes.

IMPORTANT: If this backport is for a servicing release, please verify that:

• For .NET 8 and .NET 9: The PR target branch is release/X.0-staging, not release/X.0.
• For .NET 10+: The PR target branch is release/X.0 (no -staging suffix).

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

Note

This backport template was completed by GitHub Copilot at the user's direction.

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Approved. Please check test failures once CI is done.