x86-64: refuse "high" 8-bit regs with .insn and VEX/XOP/EVEX encodings

Much like REX, those encodings - if permitting 8-bit regs at all, i.e.
only starting with APX - permit use of "new" 8-bit registers only. %ah,
%ch, %dh, and %bh cannot be encoded and hence should be rejected.

Permit their use outside of 64-bit code though, as "new" registers
simply don't exist there.