Save one syscache lookup when examining volatility or strictness of

OpExpr and related nodes.  We're going to have to set the opfuncid of
such nodes eventually (if we haven't already), so we might as well
exploit the opportunity to cache the function OID.  Buys back some
of the extra planner overhead noted by Guillaume Smet, though I still
need to fool with equivclass.c to really respond to that.