llvm-mc: Match a few X86 instructions.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77657 91177308-0d34-0410-b5e6-96231b3b80d8