AMDGPU/GlobalISel: Implement select for 32-bit G_ADD

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: hiraditya, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58804

llvm-svn: 364797