Expose AVX512 Compare and BlendVariable APIs

[tannergooding]

This makes progress towards #87097

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Issue Details

---

This makes progress towards #87097

Author:	tannergooding
Assignees:	-
Labels:	area-CodeGen-coreclr, new-api-needs-documentation
Milestone:	-

[tannergooding]

CC. @dotnet/jit-contrib, @dotnet/avx512-contrib

This is the first of the APIs that required VectorMask under the old design and which abstract that away in lowering instead under the new design.

There should be 2-3 more PRs to finish out the public API surface for them. That will cover Gather/Scatter, Compress/Expand, and MoveMask/MaskStore/MaskLoad.

That will then finish the public API surface for the new design. There would still be further possible work, but it would be just polish/light-up in lowering to better handle the vector mask like scenarios.

[tannergooding]

This is still ready for review, just merging in upstream since its been a couple days since CI completed.