[globalisel] Move as much RegisterBank initialization to the constructor as possible

Summary:
The register bank is now entirely initialized in the constructor. However,
we still have the hardcoded number of register classes which will be
dealt with in the TableGen patch (D27338) since we do not have access
to this information to resolve this at this stage. The number of register
classes is known to the TRI and to TableGen but the RegisterBank
constructor is too early for the former and too late for the latter.
This will be fixed when the data is tablegen-erated.

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

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

Differential Revision: https://reviews.llvm.org/D27809

llvm-svn: 291770