[llvm] [VP] Mark llvm.experimental.vp.splice as having no functional equivalent (PR #70647)

[Luke Lau via llvm-commits]

lukel97 wrote:

> So would we be better off removing it until we have an actual use case?

By "it" do you mean removing the `VP_PROPERTY_FUNCTIONAL_INTRINSIC` property? Or the actual experimental.vp.splice intrinsic? 

https://github.com/llvm/llvm-project/pull/70647