For decoding, keep the current behavior of always decoding these as their REP
versions. In the future, this could be improved to recognize the cases where
these behave as XACQUIRE and XRELEASE and decode them as such.

llvm-svn: 184207

llvm-svn: 178083

llvm-svn: 177888

rdar://13318048

llvm-svn: 176828

- Add RTM code generation support throught 3 X86 intrinsics:
  xbegin()/xend() to start/end a transaction region, and xabort() to abort a
  tranaction region

llvm-svn: 167573