AMDGPU: Add llvm.amdgcn.ds.ordered.add & swap

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D52944

llvm-svn: 351351