[AMDGPU] Add test

Summary:
Add test checking that the redundant immediate MOV instruction
(by-product of handling phi nodes) is not found in the generated code.

Reviewers: arsenm, anton-afanasyev, craig.topper, rtereshin, bogner

Reviewed By: arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63860

llvm-svn: 370634