`opt_dead_node_elimination' are set.
The graph may not be in state phase_building. The outs datasturcture
is freed, the outs state set to no_outs. (@@@ Change this? -> inconsistent.)
+ Backedge information is conserved.
Removes old attributes of nodes. Sets link field to NULL.
Attention: the numbers assigned to nodes if the library is compiled for
development/debugging are not conserved by copying. */