GlobalISel: Relax handling of G_ASSERT_* with source register classes

The most common situation where G_ASSERT_ZEXT appears for AMDGPU is a
copy from a physical register, which happens to use set the actual
register class on the virtual register. After copy coalescing, the
assert's source operand had a vreg with a set class. The verifier was
strictly rejecting cases where the set class/bank weren't an exact
match. Additionally, RegBankSelect was also expecting a register bank
to be set on the register, not a class.

This is much stricter than regular copies so relax this behavior. This
now allows these 2 cases:

1. Source register has either class or bank, and the result does not
2. Source register has a register class, and the result is a register
with a matching bank.

This should avoid needing some kind of special handling to avoid
violating this constraint when folding copies.