AMDGPU: Make fixing i1 copies robust against re-ordering

Summary:
The new test case led to incorrect code.

Change-Id: Ief48b227e97aa662dd3535c9bafb27d4a184efca

Reviewers: arsenm, david-salinas

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63871

llvm-svn: 364566