[X86] Allow combinevxi1Bitcast to use pmovmskb on avx512 targets if the input is...

[X86] Allow combinevxi1Bitcast to use pmovmskb on avx512 targets if the input is a truncate from v16i8/v32i8.

This is especially helpful on targets without avx512bw since we don't have a good way to convert from v16i8/v32i8 to v16i1/v32i1 for the truncate anyway. If we're just going to convert it to a GPR we might as well use pmovmskb to accomplish both.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350480 91177308-0d34-0410-b5e6-96231b3b80d8