ARM: Fix cmpxchg O0 expansion

This is the equivalent of r304048 for ARM:

- 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.

llvm-svn: 304267