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

[Luke Lau via llvm-commits]

lukel97 wrote:

> Should we just drop handling for it for now or is it possible to create a test case?

We don't currently handle it in ExpandVectorPredication.cpp to begin with, but I guess we might in future?
I ran through all the users of `VPIntrinsic::getFunctionalOpcode()`/`VPIntrinsic::getFunctionalIntrinsicID()`/`VPIntrinsic::getConstrainedIntrinsicID()` anyway. Looks like the only users are ExpandVectorPredication.cpp and the `scalarizeVPIntrinsic` combine in VectorCombine.cpp, but the latter only operates on binary ops so it would never have triggered to begin with. So I don't think it's possible to write a failing test case currently.

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