[llvm] s390x: pattern match saturated truncation (PR #155377)

[Folkert de Vries via llvm-commits]

folkertdev wrote:

found the issue: after some copy/pasting the literal value for min and max was the same. That's not right. Also to match on signed values, the pattern must use `Imm.getSExtValue()`, otherwise that match fails.

Now I'll just need to generalize this a bit.

https://github.com/llvm/llvm-project/pull/155377