[GISel] Remove BitVector from RegBank. Use tablegen CoverageData tables directly. NFC (#71105)

RegBanks are allocated as global variables. The use of BitVector causes
a static global constructor to be used. The BitVector is initialized
from a table of bits that is created by tablegen. We can keep a pointer
to that data and use it as the bit vector instead.

This does require a little bit of manual indexing and reimplementation
of BitVector::count.