[llvm] [RISCV] Combine vp_strided_load with zero stride to scalar load + splat (PR #97798)

[Luke Lau via llvm-commits]

lukel97 wrote:

> LGTM in general, but I think we should wait for your PR for removing riscv_strided_load/store?

This is a dependency to remove `riscv_strided_load/store`, since we get some small regressions if we switch to vp.strided.load/store without handling this case

https://github.com/llvm/llvm-project/pull/97798