[AMDGPU] Fix DPP operand order in atomic optimizer

Summary:
Ensure order of operands in DPP atomic optimizer final WWM step is appropriate for sub instructions.

Change-Id: I631d050e1c00a3b4bc7c11a90437064403c4cf30

Reviewers: sheredom, tpr

Reviewed By: sheredom

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58900

llvm-svn: 355394