[llvm] [SystemZ] Enable AtomicExpand pass (PR #70398)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

Thanks, this looks good to me.  Could you also add test cases for the newly-supported operations?   Along the lines of the existing tests in llvm/test/CodeGen/SystemZ/atomicrmw-\*, but for fadd/fsub/fmin/fmax and potentially uinc_wrap/udec_wrap.   Also, maybe add variants of the atomic-load/store-* and atomicrmw-xchg-* tests for floating-point types.

https://github.com/llvm/llvm-project/pull/70398