AMDGPU/GlobalISel: Mark 32-bit G_ADD as legal

Reviewers: arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D33992

llvm-svn: 305232