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

[Nathan Corbyn via llvm-commits]

cofibrant wrote:

We've had a new failure in one of the stage 1 builds (https://green.lab.llvm.org/job/llvm.org/job/clang-stage1-cmake-RA-incremental/12880/) and it looks like this commit could be the cultist. @lukel97, could you verify that this change doesn't impact [LLVM.CodeGen/RISCV.rvp-ext-rv32.ll](https://green.lab.llvm.org/job/llvm.org/job/clang-stage1-cmake-RA-incremental/12880/testReport/junit/LLVM/CodeGen_RISCV/rvp_ext_rv32_ll/)? 

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