[GlobalISel][AArch64] Fix miscompile caused by wrong G_ZEXT selection in GISel

The miscompile case's G_ZEXT has a G_FREEZE source.  Similar to D127154, this patch removed isDef32, relying on the AArch64MIPeephole optimizer to remove redundant SUBREG_TO_REG nodes also in GISel.

Fix #58431

Reviewed By: paquette

Differential Revision: https://reviews.llvm.org/D136433