[llvm] [RISCV][TTI] Implement instruction cost for vp.reduce.* (PR #114184)

[Philip Reames via llvm-commits]

https://github.com/preames requested changes to this pull request.

I am not sure the fully duplicated test coverage for vp.reduce is justified here.  The whole point of this change is that they're cost just like normal reduces?  Why do we need to complete double all test coverage?

https://github.com/llvm/llvm-project/pull/114184