[AMDGPU] Fix the new atomic optimizer in pixel shaders.

The new atomic optimizer I previously added in D51969 did not work
correctly when a pixel shader was using derivatives, and had helper
lanes active.

To fix this we add an llvm.amdgcn.ps.live call that guards a branch
around the entire atomic operation - ensuring that all helper lanes are
inactive within the wavefront when we compute our atomic results.

I've added a test case that can cause derivatives, and exposes the
problem.

Differential Revision: https://reviews.llvm.org/D53930

llvm-svn: 346128