Remove a way to make the simplifier go into an
infinite loop.   This has been there for some weeks;
and George's UniForm tickled it.  I'm amazed nothing
else has done so.  I'll add a test.

The bad case happened when you go

	let xs = 1:xs
	in
	foldr k z xs

Then we kept firing the foldr/cons rule.

Solution: we don't attach an unfolding *at all* to
loop breakers (Simplify.completeBinding)