TableGen/GlobalISel: Don't reconstruct CodeGenRegBank

The maps for dealing with the relationships between different register
classes and subregister indexes rely on unique pointers for every
class/index. By constructing a second copy of CodeGenRegBank, two
different pointer values existed for a given subregister depending on
where you were querying.

Use the existing CodeGenRegBank owned by the CodeGenTarget instead of
constructing a second copy. This avoids incorrectly failing map
lookups in a future change.