Use change_flags consitently.

theano/compile/debugmode.py

@@ -2228,17 +2228,11 @@ class _Maker(FunctionMaker):  # inheritance buys a few helper functions
                 inputs, outputs, accept_inplace)
             fgraph.equivalence_tracker = equivalence_tracker
 
-            # optimize the fgraph
-            compute_test_value_orig = theano.config.compute_test_value
-            try:
-                theano.config.compute_test_value = \
-                    theano.config.compute_test_value_opt
+            with change_flags(compute_test_value=config.compute_test_value_opt):
                 optimizer(fgraph)
 
                 theano.compile.function_module.insert_deepcopy(
                     fgraph, inputs, list(chain(outputs, additional_outputs)))
-            finally:
-                theano.config.compute_test_value = compute_test_value_orig
 
             if i == 0:
                 fgraph0 = fgraph
@@ -2287,7 +2281,7 @@ class _Maker(FunctionMaker):  # inheritance buys a few helper functions
             fgraph.attach_feature(gof.DestroyHandler())
         for o in fgraph.outputs:
             try:
-                with change_flags(compute_test_value='off'):
+                with change_flags(compute_test_value=config.compute_test_value_opt):
                     fgraph.replace_validate(o, _output_guard(o), reason='output_guard')
                 raise Exception("Output variable %s required output_guard, "
                                 "how was this output left unprotected against "