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

In the existing version, SystemZTTIImpl::shouldExpandReduction will
create a `cast` error when handling vector reduction intrinsics that
do not have the vector to reduce as their first operand, such as
`llvm.vector.reduce.fadd` and `llvm.vector.reduce.fmul`.
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.