[llvm] [RISCV] Remove last use of @llvm.experimental.vp.splat in RISCVCodeGenPrepare. NFCI (PR #170543)

[Luke Lau via llvm-commits]

lukel97 wrote:

@cofibrant I think that failure came from #170399, but it looks like it was fixed in 9c60d70df9212734d0cf332b4b687a4942cc7257. Have the stage 1 builds gone back to green yet?

https://github.com/llvm/llvm-project/pull/170543