[x86] Make AsmParser validate registers for memory operands a bit better

We can't do a perfect job here. We *have* to allow (%dx) even in 64-bit
mode, for example, because it might be used for an unofficial form of
the in/out instructions. We actually want to do a better job of validation
*later*. Perhaps *instead* of doing it where we are at the moment.

But for now, doing what validation we *can* do in the place that the code
already has its validation, is an improvement.

llvm-svn: 198760