`substitute_node`: Should it be allowed to substitute with a dead node?

[lee30sonia]

The quality test (to be more specific, "Test quality improvement of MIG cut rewriting with compatibility graph and NPN4 resynthesis") does not pass anymore after the fix of #560. This is because cut_rewriting_with_compatibility_graph tries to substitute with a dead node and "revive" it. There are a few possible fixes:

• Remove tests for cut_rewriting_with_compatibility_graph, as we already know this algorithm is buggy and is not recommended to use.
• Modify cut_rewriting_with_compatibility_graph to skip the substitution if the replacement is dead. This will affect the quality of cut_rewriting_with_compatibility_graph, so the expected number in the quality test will also be modified. Also, is_dead needs to be implemented for klut_network.
• Modify cut_rewriting_with_compatibility_graph to use substitute_nodes. For this, substitute_nodes needs to be implemented for other network types than AIG. With a quick experiment, this fix also affected the quality of cut_rewriting_with_compatibility_graph, and ended up in an infinite while loop in "Test quality improvement of cut rewriting with AIG NPN4 resynthesis", meaning that there might be some other problems...
• Correctly deal with the case where the new signal is dead in substitute_node to "revive" it. This will affect other networks and algorithms, but also makes sense to some extent. So the question is: should this be allowed?

[lee30sonia]

#567 will fix the issue with the second solution above. However, I will keep this issue open as the other possibilities (especially the last one) could still be discussed.

[lee30sonia]

#586 fixes this issue better with revive_node (last option above). Changes related to cut_rewriting_with_compatibility_graph (including quality test) in #567 has been reverted.