AMDGPU: Initialize GlobalISel passes

Summary:
This fixes AMDGPU GlobalISel test failures when enabling the AMDGPU
target without any other targets that use GlobalISel.

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D45353

llvm-svn: 329588