AMDGPU: Uniform branch conditions can originate with intrinsics

Summary:
Discovered by Dave Airlie, fixes an assertion in Khronos OpenGL CTS
GL43-CTS.shader_storage_buffer_object.advanced-matrix.

In this particular case, the buffer load intrinsic fed into a uniform
conditional branch, and led the brcond lowering down the wrong path.

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D19931

llvm-svn: 268650