x86: fold AVX512-VNNI disassembler entries with AVX-VNNI ones

Make %XV also print the separating blank in the VEX case, while making
it do nothing for EVEX-encoded insns. This way the AVX-VNNI entries
can be re-used for AVX512-VNNI, at the same time fixing the lack of
EVEX.W decoding.

For the AVX-VNNI ones further make sure only VEX.66 forms are actually
decoded.