[llvm] [RISCV][CostModel] Updates reduction and shuffle cost (PR #77342)

[Philip Reames via llvm-commits]

preames wrote:

I split one piece of this off in https://github.com/llvm/llvm-project/pull/77963.

For this to make rapid progress, I recommend splitting it into individual pieces to the maximum extent possible.  

https://github.com/llvm/llvm-project/pull/77342