Don't perform final simplification of unstable unfoldings

In #20364 @AndreasK noted one of the unfortunate consequences of our willingness to simplify unstable unfoldings: wrappers, which inline during the final phase, are inlined. As noted by #20364, this may break rules.

@mpickering pointed out an easy way to avoid this: don't perform final-phase simplifications on unstable unfoldings. In principle this makes sense: in the case of an unfolding the "final" phase of simplifications are not the final simplifications that the expression will see; further simplification will occur after the unfolding is inlined into an occurrence site.