[VP] Add more cast VPintrinsic and docs.

Add vp.fptoui, vp.uitofp, vp.fptrunc, vp.fpext, vp.trunc, vp.zext, vp.sext, vp.ptrtoint, vp.inttoptr intrinsic and docs.

Reviewed By: frasercrmck, craig.topper

Differential Revision: https://reviews.llvm.org/D122291

Author: NexMing