AArch64/GlobalISel: Reduce use of getMinClassForRegBank

getMinClassForRegBank and getRegClassForTypeOnBank were basically
identical functions with different APIs. Consolidate on the version
that uses LLT instead of a bitwidth, since that would be more
appropriate to use in a generic API. Keep getMinClassForRegBank around
for now, since copies are a special case that can't simply read the
type from the register operands.