[AArch64] Teach the subtarget how to get to the RegisterBankInfo.

Rework the access to GlobalISel APIs to contain how much of
the APIs we need to access for the final executable to build when
GlobalISel is not built.

This prevents massive usage of ifdefs in various places. Now, all the
GlobalISel ifdefs will be happing only in AArch64TargetMachine.cpp.

llvm-svn: 265567