[llvm] [IR][RISCV] Remove @llvm.experimental.vp.splat (PR #171084)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Do we need to add AutoUpgrade support for compatibility with old bitcode? I don't know if the "experimental" allows us to skip that if it was used by an in-tree pass.

It was only emitted from RISCVCodeGenPrepare in the backend though. Do we ever serialize to bitcode at that point?

https://github.com/llvm/llvm-project/pull/171084