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

[Ulrich Weigand via llvm-commits]

uweigand wrote:

Let's wait until we have agreement whether or not misaligned atomic load/store is supposed to be supported at the IR level.  If yes, we should test the relevant cases for us: which would mean both 8-byte and 16-byte aligned 128-bit accesses.

Also, the related test case improvement from https://github.com/llvm/llvm-project/pull/75879 should be pulled in here: missing checks for serialization, test all 128-bit operations on both z10 and z13.

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