[LangRef][VP] Document vp.load and vp.store intrinsics

This patch fleshes out the missing documentation for two of the VP
intrinsics introduced in D99355: `llvm.vp.load` and `llvm.vp.store`. It
does so mostly by deferring to the `llvm.masked.load` and
`llvm.masked.store` intrinsics, respectively.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D112930