Make the intent clearer in the final GC pass.

theano/gof/vm.py

@@ -408,7 +408,7 @@ class Stack(VM):
         # lazy evaluation. See discussion on theano-dev June 19 2012.
         if self.allow_gc:
             for v in storage_map:
-                if v.owner and 'output' not in zip(*v.clients)[0]:
+                if v.owner and not v in self.outputs:
                     storage_map[v][0] = None
                     compute_map[v][0] = 0