[AVX512] Add patterns for VEXTRACT v16i16->v8i16 and v32i8->v16i8. Disable AVX2 versions of vector extract when AVX512VL is enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270318 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed