Eliminate asm parser's dependency on TargetMachine:

- Each target asm parser now creates its own MCSubtatgetInfo (if needed).
- Changed AssemblerPredicate to take subtarget features which tablegen uses
  to generate asm matcher subtarget feature queries. e.g.
  "ModeThumb,FeatureThumb2" is translated to
  "(Bits & ModeThumb) != 0 && (Bits & FeatureThumb2) != 0".

llvm-svn: 134678