Move TableGen's register bank classes to their own source file.

Move TableGen's register bank classes to their own source file.

I'll be moving some more code there to gather all of the
register-specific stuff in one place. Currently it is shared between
CodeGenTarget and RegisterInfoEmitter.

The plan is that CodeGenRegisters can compute the full register bank
structure while RegisterInfoEmitter only will handle the printing part.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132788 91177308-0d34-0410-b5e6-96231b3b80d8