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

[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.

llvm-svn: 350480