x86: VexVVVV is now merely a boolean

With the SDM long having dropped the NDS/NDD/DDS concept of identifying
encoding variants, we can finally do away with this concept as well. Of
the few consumers of the attribute, only an assertion was still checking
for a particular value, which we don't really need to retain.

When touching lines anyway, modernize other aspects as well. This often
improves similarity to adjacent lines.