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

[Yeting Kuo via llvm-commits]

yetingk wrote:

#98579 causes crash when the type of vp.stride is pointer vector type, since pointer types are unexpected for getEVT. 

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