Change the behavior of mkNodeGraph to tacitly ignore all instances
where one of the nodes is not in the list of nodes. In this way, we
can construct sub-graphs by filtering the nodes and ignoring any
possibly added isolated nodes for the missing indexes.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>