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

[Ulrich Weigand via llvm-commits]

uweigand wrote:

The IR specs say: "align must be explicitly specified on atomic loads, and the load has undefined behavior if the alignment is not set to a value which is at least the size in bytes of the pointee" -- given that, does it really make sense to add test cases that are undefined IR according to the spec?

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