Make an opt don't add useless node that futur opt would remove.

theano/tensor/opt.py

@@ -3251,6 +3251,9 @@ def local_IncSubtensor_serialize(node):
         if movable_inputs:
             new_inputs = ([i for i in node.inputs if not movable(i)] +
                           [mi.owner.inputs[0] for mi in movable_inputs])
+            if len(new_inputs) == 0:
+                new_add = new_inputs[0]
+            else:
                 new_add = T.add(*new_inputs)
 
                 # Copy over stacktrace from original output, as an error