[llvm] [SystemZ] Use getSignedConstant() where necessary (PR #117181)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

I guess these changes look fine - the more interesting question is, does this catch *all* instances of `getConstant` that need to be updated?   There's a lot more of those in the back-end ...

https://github.com/llvm/llvm-project/pull/117181