Improve the loop-breaking heuristics

Improve the loop-breaking heuristics

The loop-breaking heuristics were making it a high priority to
avoid choosing a variable as a loop breaker if its *type* was a
data type.  The reason is that it's very good to be able to "see"
constructor applications.

But it's only good if the constructor application is *visible*,
so that is what I test for now.  I found a case (when testing
SpecConstr) where I had a Rec like this:
	rec { lvl = foo Nothing
	      foo = ...
	        RULE foo Nothing = ...
   	    }

Even if lvl has a data type, it's much better to make lvl the loop
breaker, not foo, so that foo's RULE is visible in lvl's RHS.