x86: drop {X,Y,Z}MMWORD_MNEM_SUFFIX

x86: drop {X,Y,Z}MMWORD_MNEM_SUFFIX

They aren't really useful (anymore?): The conflicting operand size check
isn't applicable to any insn validly using respective memory operand
sizes (and if they're used wrongly, another error would result), and the
logic in process_suffix() can be easily changed to work without them.

While re-structuring conditionals in process_suffix() also drop the
CMPXCHG8B special case in favor of a NoRex64 attribute in the opcode
table.