Id->slot0
(id->slot0 id aignet) fetches the slot0 data for the given node from the
fanin node array.
This field includes the type code (accessible with snode->type
and a 30-bit literal accessible via snode->fanin: either the fanin0 of a
gate or the next-state literal of a register. Primary input and constant nodes
have 0 in this field.
