x86: simplify decode of opcodes valid with (embedded) 66 prefix only

x86: simplify decode of opcodes valid with (embedded) 66 prefix only

The only valid (embedded or explicit) prefix being the data size one
(which is a fairly common pattern), avoid going through prefix_table[].
Instead extend the "required prefix" logic to also handle PREFIX_DATA
alone in a table entry, now used to identify this case. This requires
moving the (adjusted) ->prefix_requirement logic ahead of the printing
of stray prefixes, as the latter needs to observe the new setting of
PREFIX_DATA in used_prefixes.

Also add PREFIX_OPCODE on related entries when previously there was
mistakenly no decode step through prefix_table[].