For DFunUnfoldings we were failing to occurrence-analyse the unfolding,
and that meant that a loop breaker wasn't marked as such, which in turn
meant it was inlined away when it still had occurrence sites.  See
Note [Occurrrence analysis of unfoldings] in CoreUnfold.

This is a pretty long-standing bug, happily nailed by John Lato.

(cherry picked from commit 87bbc69c)