Id->slot1
(id->slot1 id aignet) fetches the slot0 data for the given node from the
fanin node array.
This field includes the regp bit (accessible with
snode->regp), the phase bit (snode->phase), and a 30-bit field that
is the fanin1 literal for a gate node (snode->fanin) or else the PI or
register number of a PI/register node (snode->ionum).
