Fix XAG constant fanin optimization (#364)
Fix XAG constant fanin optimization (#364)

* Use std function to compute the symmetric difference.

* Fix bug tha was wrongly connecting nodes when the lfi of the parent was one.

* No need to cleanup the returning network twice.

* Test case for XAG constant fanin optimization.

Co-authored-by: Heinz Riener <348baae8f0a5be32cfbdd40cad49c5d81f88f0c2@gmail.com>
lsils/mockturtle,lsils/mockturtle,lsils/mockturtle,lsils/mockturtle