[llvm] [SystemZ] Handle IR struct arguments correctly. (PR #169583)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand commented:

See inline comments - something is weird with test cases, not sure what's going on here.

It would be good to also test the case where an LLVM IR struct is *returned*.   Also, the same issue would apply to LLVM IR arrays, so some tests for that would also be good.

https://github.com/llvm/llvm-project/pull/169583