[AArch64][GlobalISel] Add a post-legalizer combiner with a very simple combine.

(This patch is by Jessica, I'm just committing it on her behalf because I need
a post-legalizer combiner for something else).

This supersedes D77250, which did equivalent work in the selector. This can be
done pre-legalization or post-legalization. Post-legalization is more likely to
hit, since G_IMPLICIT_DEFs tend to appear during legalization. There's no reason
to not do it pre-legalization though-- if it can be caught earlier, great.

(I also think that it might be worth reimplementing D78769 using a
target-specific post-legalization combine too after thinking about it for a
while.)

Differential Revision: https://reviews.llvm.org/D78852