[llvm] [SystemZ] Add realistic cost estimates for vector reduction intrinsics (PR #116602)

[Dominik Steenken via llvm-commits]

dominik-steenken wrote:

Reopening this as `main` has now adopted https://github.com/llvm/llvm-project/pull/112491, which this PR was waiting on.

I am still waiting on final confirmation from the benchmarks.

https://github.com/llvm/llvm-project/pull/116602