[AVX512] Use MOVZX32 instead of MOVZ16 for loading single v8/v4/v2/v1 masks...

[AVX512] Use MOVZX32 instead of MOVZ16 for loading single v8/v4/v2/v1 masks when KMOVB is not available. This has better behavior with respect to partial register stalls since it won't need to preserve the upper 16-bits of the GPR.

llvm-svn: 272626