AMDGPU/SI: Add llvm.amdgcn.v.interp.p[12] intrinsics

Summary:
These are meant to be used instead of the llvm.SI.fs.interp intrinsic which
will be deprecated at some point.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15474

llvm-svn: 255651