Enable V3DfgCache to look up vertices without creating one. Reuse
existing terms in associative expression trees if they already exist
somewhere in the graph.
Note this might miss some cases where a sub-tree within an And/Or/Xor
tree is optimizeable, but not the whole tree, but in practice this seems
to work better than the alternative of keeping a set of failed nodes and
bail early.
Geza Lore
Wilson Snyder
Geza Lore