- Eliminate MCCodeEmitter's dependency on TargetMachine. It now uses MCInstrInfo

- Eliminate MCCodeEmitter's dependency on TargetMachine. It now uses MCInstrInfo
  and MCSubtargetInfo.
- Added methods to update subtarget features (used when targets automatically
  detect subtarget features or switch modes).
- Teach X86Subtarget to update MCSubtargetInfo features bits since the
  MCSubtargetInfo layer can be shared with other modules.
- These fixes .code 16 / .code 32 support since mode switch is updated in
  MCSubtargetInfo so MC code emitter can do the right thing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134884 91177308-0d34-0410-b5e6-96231b3b80d8