[AVX512] Peel off an asm-only class from AVX512_masking_common.
[AVX512] Peel off an asm-only class from AVX512_masking_common.

No functional change.

This enables the generation of masking instructions that don't provide a
ISel pattern.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@219358 91177308-0d34-0410-b5e6-96231b3b80d8

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