AMDGPU/GlobalISel: legalize and select 32-bit G_ASHR

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D48196

llvm-svn: 335318