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

[Philip Reames via llvm-commits]

preames wrote:

I've made specific review suggestions you have not applied.  As a means to make forward progress here, are you okay with me landing a modified version of this patch?  The code is fine, it's only the testing bits we're stuck on.  I'll simply make the testing change I've requested, and you can revisit testing improvements in a separate review if you want.

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