CNF Encoded Hard Miters from IWLS'22 Paper

Five Hard Miters from an IWLS'22 Paper Encoded into CNF in two Ways

The IWLS'22 paper [1] by He-Teng Zhang, Jie-Hong R. Jiang, Alan Mishchenko,
and Luca Amaru, which can be considered an update on their DAC'21 [2] paper
was focusing on a hybrid approach to SAT-sweeping, i.e., using a SAT solver
incrementally, taking the circuit structure into account.

Experiments in [1] used a subset of the benchmarks from [2] which also
contains five miters `n01`, `n04`, `n06`, and `test01` and `test02` provided
to us by Alan Mishchenko and which were considered hard for SAT sweeping,
and particularly for monolithic CNF-level SAT solving.

  [1] H.-T. Zhang, J.-H. R. Jiang, A. Mishchenko, and L. Amaru.  "Improved
  large-scale SAT sweeping", In Proc. Intl. Work. on Logic Synthesis
  (IWLS'22).

  [2] H.-T. Zhang, J.-H. R. Jiang, L. Amaru, A. Mishchenko, and R. Brayton.
  "Deep integration of circuit simulator and SAT solver". In Proc. of Design
  Automation Conference (DAC'21), 2021.

It turned out, confirmed by Alan Mishchenko, that the outputs of `test01`
and `test02`, were flipped in the process they were generated.  This does
not invalidate the SAT sweeping experiments in [1] and [2] at all, but needs
to be taken care off, when encoding them into CNF with `aigtocnf`, by simply
first negating the outputs with `aigflip`.  Further note, that the other
three AIGs `n01`, `n04`, and `n06` are not negated but have multiple outputs
and thus we joined them by disjunction with `aigor`.

We have extended `aigtocnf` to detect XOR and ITE gates in AIGER circuits,
i.e., during Tseitin encoding we check whether and an AND gate has two
negated AND gates as children and actually implements an XOR or ITE gate.
In this case we use a direct CNF encoding of four clauses, instead of 9
clauses for three AND gates, skipping the two child AND gates of the top AND
gate.  This reduces not only the number of clauses and variables but has
positive effects on solving time.  Thus all the CNF benchmarks come in two
flavors tagged `xits` with special treatment of XOR and ITE gates during
Tseitin encoding and `ands` without.