[aarch64][globalisel] Refactor getRegBankBaseIdxOffset() to remove the power-of-2 assumption. NFC

Summary:
We don't exploit it yet though

Depends on D27976

Reviewers: t.p.northover, ab, rovka, qcolombet

Subscribers: aditya_nandakumar, aemerson, rengolin, vkalintiris, dberris, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D27977

llvm-svn: 291899