Fix(print_mffc): Missing condition when single output linked to CO

[wjrforcyber]

Bug

print_mffc can't print some simple examples.

Minimum Reproducible Example (MRE)

Given aig

aag 5 3 0 1 2
2
4
6
10
8 2 4
10 8 6

which looks like:

run abc with following commands:

./abc -c "read_aiger noreconv.aig; ps; print_mffc"
ABC command line: "read_aiger noreconv.aig; ps; print_mffc".

noreconv                      : i/o =    3/    1  lat =    0  and =      2  lev =  2

No mffc statistic is printed.

Analysis

The condition only considers dag node while missing the case when the node is a tree node but directly connected to PO.

Fix

After adding the corner case condition, the result will show the mffc in the circle.

./abc -c "read_aiger noreconv.aig; ps; print_mffc"
ABC command line: "read_aiger noreconv.aig; ps; print_mffc".

noreconv                      : i/o =    3/    1  lat =    0  and =      2  lev =  2
Node =     n6 : Supp =   3  Cone =   2  ( n5 n6 )