[llvm] [SystemZ] Test improvements for atomic load/store instructions (NFC). (PR #75630)

[Jonas Paulsson via llvm-commits]

JonPsson1 wrote:

> > If there is library support for the unaligned cases, I guess the libcalls should be allowed. If not, the libcalls should not get emitted by AtomicExpand.
> 
> Yes; the non-size-specialized `__atomic_*` libcalls work on any alignment. AtomicExpandPass's behavior here is correctly/intentionally handling that condition. The IR spec wording should be updated to relax the constraint.

I made a suggestion here for the spec: https://github.com/llvm/llvm-project/pull/75871

https://github.com/llvm/llvm-project/pull/75630