AMDGPU: Move isSDNodeSourceOfDivergence() implementation to SITargetLowering

Summary:
The code that handles ISD:Register and ISD::CopyFromReg assumes
the target is amdgcn, so this is broken on r600.  We don't
need this analysis on r600 anyway so we can safely move
it to SITargetLowering.

Reviewers: alex-t, arsenm, nhaehnle

Reviewed By: arsenm

Subscribers: msearles, kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D46298

llvm-svn: 334607