AMDGPU: Fix lit test checks with dag option

Summary:
I was seeing some failures on a test with slightly different instruction
ordering. Adding in some DAG directives solved the issue.

Change-Id: If5a3d3969055fb19279943bd45161bb70a3dabce

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70531