[llvm] [RISCV][TTI] Add shuffle costing for masked slide lowering (PR #128537)

[Philip Reames via llvm-commits]

preames wrote:

Ok, review comments addressed.  Sorry for the very messy commits, I usually rebase and squash, but managed to push first this time.  I rewrote the history, so hopefully that's a bit easier to follow.

https://github.com/llvm/llvm-project/pull/128537