AMDGPU/SI: Implement a work-around for smrd corrupting vccz bit

Summary:
We will hit this once we have enabled uniform branches.  The
smrd-vccz-bug.ll test will be added with the uniform branch commit.

Reviewers: mareko, arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D16725

llvm-svn: 260137