llvm-mc: Match a few X86 instructions.

 - This is "experimental" code, I am feeling my way around and working out the
   best way to do things (and learning tblgen in the process). Comments welcome,
   but keep in mind this stuff will change radically.

 - This is enough to match "subb" and friends, but not much else. The next step is to
   automatically generate the matchers for individual operands.

llvm-svn: 77657