AMDGPU/SI: Move some ISel helpers into utils so they can be shared with GISel

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, llvm-commits, tony-tye

Differential Revision: https://reviews.llvm.org/D29068

llvm-svn: 293321