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

[James Y Knight via llvm-commits]

jyknight 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.

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