Implement SPARCv9 atomic_swap_64 with a pseudo.

The SWAP instruction only exists in a 32-bit variant, but the 64-bit
atomic swap can be implemented in terms of CASX, like the other atomic
rmw primitives.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200453 91177308-0d34-0410-b5e6-96231b3b80d8