x86: improve special casing of certain insns

x86: improve special casing of certain insns

Now that we have identifiers for the mnemonic strings we can avoid
opcode based comparisons, for (in many cases) being more expensive and
(in a few cases) being a little fragile and not self-documenting.

Note that the MOV optimization can be engaged by the earlier LEA one,
and hence LEA also needs checking for there.