[llvm] [RISCV][TTI] Add llvm.vp.select into canSplatOperand. (PR #117982)

[Luke Lau via llvm-commits]

lukel97 wrote:

Thanks. Maybe add a bit in the PR description to mention that this allows us to select vmerge.vxm?

https://github.com/llvm/llvm-project/pull/117982