AMDGPU/SI: Fix implemenation of isSourceOfDivergence() for graphics shaders

Summary:
The analysis of shader inputs was completely wrong.  We were passing the
wrong index to AttributeSet::hasAttribute() and the logic for which
inputs where in SGPRs was wrong too.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15608

llvm-svn: 256082