x86: RMPUPDATE wants operands in different form

x86: RMPUPDATE wants operands in different form

AMD are about to update their doc, to help clarify that what we
currently do isn't quite right: In particular it is not %rax but %rcx
which is affected by address size. In fact, that's a normal memory
operand, just not expressed via ModR/M byte, but fixed to (%rcx) (or
(%ecx) with 32-bit addressing).

To support this in the assembler, generalize memory operand handling so
far specific to XLAT (which isn't really a string insn, but requires its
memory operand to be (%bx) / (%ebx) / (%rbx)).

In the disassembler mimic handling after XLAT's, too.