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

[Dominik Steenken via llvm-commits]

dominik-steenken wrote:

I thought i did - i changed the reference to a loop from earlier to
```
This commit fixes that problem by moving the cast into the case
statement that handles the specific intrinsic, where the vector
operand position is well-known.
```
do you think that's not accurate?

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