Fix Trac #4917: try a bit harder to unify on-the-fly

Fix Trac #4917: try a bit harder to unify on-the-fly

This is generally a modest improvement but, more important,
it fixes a "unify-under-forall" problem.  See Note [Avoid deferring].

There's still a lurking unsatisfactory-ness in that we can't
defer arbitrary constraints that are trapped under a forall.