Fix the code that adds regclass constants to a plan's list of relation OIDs

that it depends on for replan-forcing purposes.  We need to consider plain OID
constants too, because eval_const_expressions folds a RelabelType atop a Const
to just a Const.  This change could result in OID values that aren't really
for tables getting added to the dependency list, but the worst-case
consequence would be occasional useless replans.  Per report from Gabriele
Messineo.