ARM: Fix CMP_SWAP expansion

Clean up after my misguided attempt in r304267 to "fix" CMP_SWAP
returning an uninitialized status value.

- I was always using tMOVi8 to zero the status register which cannot
  encode higher register numbers and llvm would silently miscompile)

- Nobody was ever looking at that status value outside the expansion.
  ARMDAGToDAGISel::SelectCMP_SWAP() the only place creating CMP_SWAP
  instructions was not mapping anything to it. (The cmpxchg status value
  from llvm IR is lowered to a manual comparison after the CMP_SWAP)

So this:
- Renames the register from "status" to "temp" it make it obvious that
  it isn't used outside the expansion.
- Remove the zeroing status/temp register.
- Keep the live-in list improvements from r304267

Fixes http://llvm.org/PR34056

llvm-svn: 310534