Registers a basic block as a site for control flow to return from this function.

        :param CodeNode return_site:     The block node that ends with a return.

def _add_return_site(self, return_site):
        """
        Registers a basic block as a site for control flow to return from this function.

        :param CodeNode return_site:     The block node that ends with a return.
        """
        self._register_nodes(True, return_site)

        self._ret_sites.add(return_site)
        # A return site must be an endpoint of the function - you cannot continue execution of the current function
        # after returning
        self._add_endpoint(return_site, 'return')