eval_const_expresisions and ScalarArrayOpExpr

Eval_const_expressions() doesn't know about ScalarArrayOpExpr. We 
simplify the arguments, but if all the arguments are booleans, we don't 
take the obvious step of replacing the whole expression with a boolean 
Const. For example:

postgres=# explain select * from foo where 1 IN (1,2,3);
                          QUERY PLAN
-------------------------------------------------------------
  Result  (cost=0.00..35.50 rows=2550 width=4)
    One-Time Filter: (1 = ANY ('{1,2,3}'::integer[]))
    ->  Seq Scan on foo  (cost=0.00..35.50 rows=2550 width=4)
(3 rows)

I would've expected that to be fully evaluated at plan-time, and 
optimized away.

That seems like an oversight. I guess that scenario doesn't happen very 
often in practice, but there's no reason not to do it when it does. 
Patch attached.

On a side-note, I find it a bit awkward that ScalarArrayOpExpr uses a 
2-element List to hold the scalar and array arguments. Wouldn't it be 
much more straightforward to have explicit "Expr *scalararg" and "Expr 
*arrayarg" fields?

- Heikki