[llvm] [RISCV][IR] Implement verifier check for llvm.experimental.vp.splice immediate. (PR #147458)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> I guess if the vectorizer is going to emit this immediate it must know that the value is in range for all possible vscale values so maybe this makes sense?

Currently we only have `imm==-1`.

https://github.com/llvm/llvm-project/pull/147458