AArch64: fix 128-bit cmpxchg at -O0 (again, again).

This time the issue is fortunately just a simple mistake rather than a horrible
design spectre. I thought SUBS/SBCS provided sufficient NZCV flags for
comparing two 64-bit values, but they don't.

The fix is slightly clunkier in AArch64 because we can't use conditional
execution to emit a pair of CMPs. Traditionally an "icmp ne i128" would map to
an EOR/EOR/ORR/CBNZ, but that uses more registers so it's easier to go with a
CSET/CINC/CBNZ combination. Slightly less efficient, but this is -O0 anyway.

Thanks to Anton Korobeynikov for pointing out the issue.

llvm-svn: 288418