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

[Ulrich Weigand via llvm-commits]

uweigand 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?

This is fine - but this is not what GitHub's "merge and squash" was suggesting as commit message.  There I saw "This commit fixes that problem by introducing a short loop to find the vector operand instead of assuming that it is the first operand." instead, not sure why.  Anyway, I've now merged using the correct message.


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