AMDGPU: test for uniformity of branch instruction, not its condition

Summary:
If a divergent branch instruction is marked as divergent by propagation
rule 2 in DivergencePropagator::exploreSyncDependency() and its condition
is uniform, that branch would incorrectly be assumed to be uniform.

Reviewers: arsenm, tstellar

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D56331

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350532 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed