llvm-mc/X86: Implement single instruction encoding interface for MC.

 - Note, this is a gigantic hack, with the sole purpose of unblocking further
   work on the assembler (its also possible to test the mathcer more completely
   now).

 - Despite being a hack, its actually good enough to work over all of 403.gcc
   (although some encodings are probably incorrect). This is a testament to the 
   beauty of X86's MachineInstr, no doubt! ;)

llvm-svn: 80234