AMDGPU: Split AMDGPUTTI into GCNTTI and R600TTI

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D47359

llvm-svn: 333605