Merging r310534:

Merging r310534:
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r310534 | matze | 2017-08-09 15:22:05 -0700 (Wed, 09 Aug 2017) | 20 lines

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
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_50@310628 91177308-0d34-0410-b5e6-96231b3b80d8