[X86][AsmParser] Keep track of whether an explicit scale was specified while...

[X86][AsmParser] Keep track of whether an explicit scale was specified while parsing an address in Intel syntax. Use it for improved error checking.

This allows us to check these:
-16-bit addressing doesn't support scale so we should error if we find one there.
-Multiplying ESP/RSP by a scale even if the scale is 1 should be an error because ESP/RSP can't be an index.

llvm-svn: 335398