AMDGPU/GlobalISel: Fix crash in regbankselect on non-power-of-2 types

Reviewers: arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D49624

llvm-svn: 338102