Merging r277500:

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r277500 | nhaehnle | 2016-08-02 12:17:37 -0700 (Tue, 02 Aug 2016) | 18 lines

AMDGPU: Track physical registers in SIWholeQuadMode

Summary:
There are cases where uniform branch conditions are computed in VGPRs, and
we didn't correctly mark those as WQM.

The stray change in basic-branch.ll is because invoking the LiveIntervals
analysis leads to the detection of a dead register that would otherwise
not be seen at -O0.

This is a candidate for the 3.9 branch, as it fixes a possible hang.

Reviewers: arsenm, tstellarAMD, mareko

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D22673

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llvm-svn: 288103