Better solution to the IN-list issue: instead of having an arbitrary cutoff,

Better solution to the IN-list issue: instead of having an arbitrary cutoff,
treat Var and non-Var IN-list items differently.  Only non-Var items are
candidates to go into an ANY(ARRAY) construct --- we put all Vars as separate
OR conditions on the grounds that that leaves more scope for optimization.
Per suggestion from Robert Haas.