[X86] 512-bit VPAVG requires AVX512BW

Fix VPAVG detection to require AVX512BW, not AVX512F for 512-bit widths,
and change associated asserts to assert in the right direction...

This fixes PR29111.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279755 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed