[llvm] [SystemZ] SIMM32 is a signed constant (PR #118634)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand approved this pull request.

LGTM.  I think this should go in even if we don't have any simple test cases on upstream.

https://github.com/llvm/llvm-project/pull/118634