[X86][AVX512] Fixed decoding of permd/permpd variable mask shuffles + enabled them for target shuffle combining

Corrected element mask masking to extract the bottom index bits (now matches the perm2 implementation but for unary inputs).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274571 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed