[llvm] [RISCV] Use experimental.vp.splat to splat specific vector length elements. (PR #101329)

[Philip Reames via llvm-commits]

preames wrote:

> Does this need to be backported to 19.x? Not sure if we are using vp_strided_load in RISCVGatherScatterOpt there. @preames @lukel97 ?

It should not be, but @luke957 please confirm.

https://github.com/llvm/llvm-project/pull/101329