Fix https://github.com/opencv/opencv/issues/15141

modules/dnn/src/tensorflow/tf_graph_simplifier.cpp

@@ -787,7 +787,7 @@ void RemoveIdentityOps(tensorflow::GraphDef& net)
         const tensorflow::NodeDef &layer = net.node(li);
         String type = layer.op();
 
-        if (type == "Identity" || type == "Dropout") {
+        if (type == "Identity" || type == "Dropout" || type == "PlaceholderWithDefault") {
             identity_ops_idx.push_back(li);
             identity_ops[layer.name()] = layer.input(0);
         }
@@ -1031,7 +1031,7 @@ void removePhaseSwitches(tensorflow::GraphDef& net)
                 }
             }
             nodesToRemove.push_back(i);
-            if (node.op() == "Merge")
+            if (node.op() == "Merge" || node.op() == "Switch")
                 mergeOpSubgraphNodes.push(i);
         }
     }

modules/dnn/test/test_tf_importer.cpp

@@ -729,6 +729,7 @@ TEST_P(Test_TensorFlow_layers, subpixel)
 TEST_P(Test_TensorFlow_layers, keras_mobilenet_head)
 {
     runTensorFlowNet("keras_mobilenet_head");
+    runTensorFlowNet("keras_learning_phase");
 }
 
 TEST_P(Test_TensorFlow_layers, resize_bilinear)