AArch64: expand cmpxchg after regalloc at -O0.

FastRegAlloc works only at the basic-block level and spills all live-out
registers. Unfortunately for a stack-based cmpxchg near the spill slots, this
can perpetually clear the exclusive monitor, which means the cmpxchg will never
succeed.

I believe the only way to handle this within LLVM is by expanding the loop
post-regalloc. We don't want this in general because it severely limits the
optimisations that can be done, so we limit this to -O0 compilations.

It's an ugly hack, and about the one good point in the whole mess is that we
can treat all cmpxchg operations in the most naive way possible (seq_cst, no
clrex faff) without affecting correctness.

Should fix PR25526.

llvm-svn: 266339