[AArch64][GlobalISel] Enable extending loads combines post-legalization.

During legalization we can end up with extends of loads, which in the case of
zexts causes us to not hit tablegen imported patterns.

The caveat here is that we don't want anyext load forming, since some variants
are illegal. This change also prevents the combine from creating any illegal
loads.

Differential Revision: https://reviews.llvm.org/D80458