patch applied (ghc): Improve the loop-breaking heuristics

[Simon Peyton Jones]

Wed Nov 29 13:24:40 PST 2006  simonpj at microsoft.com
  * 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.
  

    M ./compiler/simplCore/OccurAnal.lhs -4 +7