[AMDGPU] Add intrinsics for 16 bit interpolation

Summary:
Added the intrinsics llvm.amdgcn.interp.p1.f16() and
llvm.amdgcn.interp.p2.f16() and related LIT test.

The p1 intrinsic generates code appropriate for both 16 and 32
bank LDS.

Reviewers: #amdgpu, dstuttard, arsenm, tpr

Reviewed By: #amdgpu, arsenm

Subscribers: jvesely, mgorny, arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D46754

llvm-svn: 352357