[AArch64][GlobalISel] Widen G_IMPLICIT_DEF and G_FREEZE before clamping

Similar to other cleanup commits which widen instructions before clamping
during legalization. Purpose of this is to avoid weird type breakdowns.

In terms of G_IMPLICIT_DEF, this simplifies legalization for other instructions.
The legalizer has to emit G_IMPLICIT_DEF to legalize certain instructions, so
this can help with emitting merges elsewhere.

Differential Revision: https://reviews.llvm.org/D107604