AMDGPU: llvm.amdgcn.interp.mov is a source of divergence

Summary:
While the result is constant across a single primitive, each pixel
shader wave can have pixels from multiple primitives.

Reviewers: tstellarAMD, arsenm

Subscribers: kzhuravl, wdng, yaxunl, llvm-commits, tony-tye

Differential Revision: https://reviews.llvm.org/D27572

llvm-svn: 289447