[nfc][codegen] Move RegisterBank[Info].cpp under CodeGen

Layering-wise, it seems RegisterBank stuff fits under CodeGen, like
other target abstraction.
In particular, TargetSubtargetInfo has a getRegBankInfo member, but
using that object requires making sure GlobalISel is linked, which is
not always the case (e.g. llvm-jitlink doesn't).

Differential Revision: https://reviews.llvm.org/D119053