Improve Smoothing Efficiency

[haz]

The following things should be done to improve the smoothing efficiency:

• Create the or(var, -var) nodes only once at the start of smoothing for lookup.
• Only create a new and(...) node during smoothing if the corresponding child is a literal and not already an and node.

Note that the second one indicates that the resulting d-DNNF will remain an And-Or graph. The attached file seems to cause a memory blow up because of new node creation, but an infinite loop is not (seemingly) present.