[llvm] [SystemZ] Fix Operand Retrieval for Vector Reduction Intrinsic in `shouldExpandReduction` (PR #88874)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

Hi @dominik-steenken I just noticed that the commit message no longer matches what the code now actually does ... can you fix this before I merge it?  Thanks!

https://github.com/llvm/llvm-project/pull/88874