Use dict as we never iterate on it.

theano/gof/destroyhandler.py

@@ -737,8 +737,8 @@ class DestroyHandler(toolbox.Bookkeeper):  # noqa
 
         self.fgraph = fgraph
         self.destroyers = OrderedSet()  # set of Apply instances with non-null destroy_map
-        self.view_i = OrderedDict()  # variable -> variable used in calculation
-        self.view_o = OrderedDict()  # variable -> set of variables that use this one as a direct input
+        self.view_i = {}  # variable -> variable used in calculation
+        self.view_o = {}  # variable -> set of variables that use this one as a direct input
         # clients: how many times does an apply use a given variable
         self.clients = OrderedDict()  # variable -> apply -> ninputs
         self.stale_droot = True