x86: correct .insn with opcode extension and VEX/XOP/EVEX encoding

When VexVVVV handling was re-worked, .insn broke: When an opcode
extension is in use, VexVVVV_DST needs using now, as ModR/M.reg is
already occupied, matching what c8866e3e ("x86: Drop using
extension_opcode to encode vvvv register") did.

While adding (bad) POP2 forms, also slightly adjust existing ones:
No need to use XMM registers, and no need to specify %r8 when really
%rax is meant twice (EVEX.vvvv not really being the culprit there, or
else EVEX.V' would also have needed mentioning).