AMDGPU/GlobalISel: Implement select() for 32-bit @llvm.minnun and @llvm.maxnum

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D46172

llvm-svn: 337056