Add a post-legalizer combiner with a very simple combine. 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.)