[llvm] [SLP]Use getExtendedReduction cost and fix reduction cost calculations (PR #117350)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM - Note that I'm only confident in the TTI bits.  The SLP bits aren't obviously broken, but that's all I can say.

https://github.com/llvm/llvm-project/pull/117350