Re-commit: [globalisel] Tablegen-erate current Register Bank Information

Summary:
Adds a RegisterBank tablegen class that can be used to declare the register
banks and an associated tablegen pass to generate the necessary code.

Changes since last commit:
The new tablegen pass is now correctly guarded by LLVM_BUILD_GLOBAL_ISEL and
this should fix the buildbots however it may not be the whole fix. The previous
buildbot failures suggest there may be a memory bug lurking that I'm unable to
reproduce (including when using asan) or spot in the source. If they re-occur
on this commit then I'll need assistance from the bot owners to track it down.

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

Reviewed By: qcolombet

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

Differential Revision: https://reviews.llvm.org/D27338

llvm-svn: 292367