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

[Nikita Popov via llvm-commits]

nikic 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 ...

It's all the instances that result in test failures :) I definitely wouldn't be surprised if there's more of these not covered by tests...

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