Fix a critical bug in 64-bit atomic operation lowering for 32-bit. The results...

Fix a critical bug in 64-bit atomic operation lowering for 32-bit. The results of the cmpxchg8b instructions are being thrown away when it branches back to the top of the checking loop. This means the loop always compares against the old value and this can result in a dead lock.

llvm-svn: 93028