[x86] Fix MOV8ao8 et al for 16-bit mode, fix up disassembler to understand

It seems there is no separate instruction class for having AdSize *and*
OpSize bits set, which is required in order to disambiguate between all
these instructions. So add that to the disassembler.

Hm, perhaps we do need an AdSize16 bit after all?

llvm-svn: 198759