[llvm] [VP][RISCV] Introduce vp.splat and RISC-V. (PR #98731)

[Luke Lau via llvm-commits]

lukel97 wrote:

> It would be quite nice to see how this is expected to help codegen where we currently use regular spats. I assume it will likely reduce `vsetvli`s? Is there a plan to introduce it to the vectorizer?

In https://github.com/llvm/llvm-project/pull/98140#discussion_r1670337004 we ran into an awkward spot where we wanted to perform a splat with an AVL to avoid vl toggles. I think the canonical form would still be shufflevector (or splat_vector for scalable vectors at the SelectionDAG level)

https://github.com/llvm/llvm-project/pull/98731