AArch64: Fix cmpxchg O0 expansion

AArch64: Fix cmpxchg O0 expansion

- Rewrite livein calculation to use the computeLiveIns() helper
  function. This is slightly less efficient but easier to reason about
  and doesn't unnecessarily add pristine and reserved registers[1]
- Zero the status register at the beginning of the loop to make sure it
  has a defined value.
- Remove kill flags of values that need to stay alive throughout the loop.

[1] An upcoming commit of mine will tighten the MachineVerifier to catch
    these.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304048 91177308-0d34-0410-b5e6-96231b3b80d8