[llvm] [SystemZ] Don't lower float/double ATOMIC_[LOAD|STORE] to [LOAD|STORE] (PR #75879)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand approved this pull request.

See inline comments, but generally this LGTM now.   As Matt suggested, I guess we should also have a `z_store` PatFrag so we keep the ATOMIC_STORE during SelectionDAG.  But this can also be looked at separately, I think.

https://github.com/llvm/llvm-project/pull/75879