[AArch64][GlobalISel] Eliminate redundant G_ZEXT when the source is implicitly zext-loaded.

These cases can come up when the extending loads combiner doesn't combine a
zext(load) to a zextload op, due to some other operation being in between, which
then gets simplified at a later stage.

Differential Revision: https://reviews.llvm.org/D65360

llvm-svn: 367723