x86: properly decode EVEX.W for AVX512_4{FMAPS,VNNIW} insns

These require EVEX.W=0. Use %XS to facilitate the checking, even if for
the AVX512_4VNNIW ones this is kind of an abuse (as 's' there stands for
"signed", not "single").

While there also correct the 3rd operand for the AVX512_4VNNIW entries:
Only the memory form is allowed (just like for AVX512_4FMAPS, where the
correct type is already in use).