AMDGPU/GlobalISel: Implement call lowering for shaders returning values

Reviewers: arsenm, nhaehnle

Subscribers: kzhuravl, jvesely, wdng, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, volkan, llvm-commits

Differential Revision: https://reviews.llvm.org/D57166

llvm-svn: 357964