'Add an edge in the data dependence graph.
:param ProgramVariable src: Source node.
:param ProgramVariable dst: Destination node.
:param edge_labels: All labels associated with the edge.
:return: None'

def _data_graph_add_edge(self, src, dst, **edge_labels):

    if ((src in self._data_graph) and (dst in self._data_graph[src])):
        return
    self._data_graph.add_edge(src, dst, **edge_labels)
    self._simplified_data_graph = None
