[GlobalISel] Implement regbankselect for G_ASSERT_ZEXT

This adds generic regbankselect support for G_ASSERT_ZEXT.

It inherits whatever register bank the source was given, always, on all targets.

I think that at the point where we run into these, the source register bank
should be decided.

This also adds some AArch64-specific code which makes sure we can handle
G_ASSERT_ZEXT when deciding on register banks for G_STORE, G_PHI, ... etc.

Differential Revision: https://reviews.llvm.org/D95649