x86: simplify decode of opcodes valid only without any (embedded) prefix

In this case there's no need to go through prefix_table[] at all - the
.prefix_requirement == PREFIX_OPCODE machinery takes care of this case
already.

A couple of further adjustments are needed though:
- Gv / Ev and alike then can't be used (needs to be Gdq / Edq instead),
- dq_mode and friends shouldn't lead to PREFIX_DATA getting set in
  used_prefixes.