[SLP]Fix the cost of the reduction result to the final type.

Need to fix the way the cost is calculated, otherwise wrong cast opcode
can be selected and lead to the over-optimistic vector cost. Plus, need
to take into account reduction type size.

Reviewers: RKSimon

Reviewed By: RKSimon

Pull Request: https://github.com/llvm/llvm-project/pull/87528