[VP] getDeclarationForParams

`VPIntrinsic::getDeclarationForParams` creates a vp intrinsic
declaration for parameters you want to call it with.  This is in
preparation of a new builder class that makes emitting vp intrinsic code
nearly as convenient as using a plain ir builder (aka `VectorBuilder`,
to be used by D99750).

Reviewed By: frasercrmck, craig.topper, vkmr

Differential Revision: https://reviews.llvm.org/D102686