x86: reduce AVX512-FP16 set of insns decoded through vex_w_table[]

Like already indicated during review of the original submission, there's
really only very few insns where going through this table is easier /
cheaper than using suitable macros. Utilize %XH more and introduce
similar %XS and %XD (which subsequently can be used for further table
size reduction).

While there also switch to using oappend() in 'XH' macro processing.