A real-world ABC9 flow hit residual combinational loops after SCC breaking, tripping the prep_xaiger loop assertion.
Keep the existing topological assertions in place (prep_xaiger and reintegrate still assert no_loops).
To handle residual non-box loops, add a targeted fallback in prep_xaiger: when loops remain after normal SCC breaking, insert additional $__ABC9_SCC_BREAKER cuts on non-box loop cells, rebuild toposort, and then re-check the existing assertion.
Also keep pre-ABC9 SCC tagging on all cell types (scc -all_cell_types) and add a regression test (tests/techmap/abc9-nonbox-loop-with-box.ys).
These updates should not be necessary. In fact, if they were necessary, this code
would be buggy, because the results would depend on the order in which wires are traversed:
If wire A is retained, which causes an update to `used_signals`, which then causes wire B
to be retained when it otherwise wouldn't be, then we would get different results depending
on whether A is visited before B.
These updates will also make it difficult to process these wires in parallel.
The negation here is confusing. The intent of the code is "if `s1` is preferred
over `s2` as the canonical `SigBit` for this signal, make `s1` the canonical `SigBit`
in `assign_map`", so write the code that way instead of "if `s2` is not preferred
over `s1` ...".
This doesn't change any behavior now that `compare_signals()` is a total order,
i.e. `s1` is preferred over `s2`, `s2` is preferred over `s1`, or `s1` and `s2` are equal.
Now, when `s1` and `s2` are equal, we don't call `assign_map.add(s1)`, but that's
already a noop in that case.
Currently when `s1` and `s2` are different bits of the same wire,
it is possible for both `compare_signals(s1, s2)` and `compare_signals(s2, s1)` to
return false. This means the calling code will call `assign_map.add()` for
both `s1` and `s2`, which doesn't make much sense --- one of `s1` or `s2`
should be consistently preferred.
So fix that by preferring the `SigBit` with the smaller bit offset.
