Teach eval_const_expressions() to simplify an ArrayCoerceExpr to a constant

when its input is constant and the element coercion function is immutable
(or nonexistent, ie, binary-coercible case).  This is an oversight in the
8.3 implementation of ArrayCoerceExpr, and its result is that certain cases
involving IN or NOT IN with constants don't get optimized as they should be.
Per experimentation with an example from Ow Mun Heng.

src/backend/optimizer/util/clauses.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.264 2008/08/25 22:42:33 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.265 2008/08/26 02:16:31 tgl Exp $
  *
  * HISTORY
  *	  AUTHOR			DATE			MAJOR EVENT
@@ -2359,6 +2359,41 @@ eval_const_expressions_mutator(Node *node,
 		newexpr->coerceformat = expr->coerceformat;
 		return (Node *) newexpr;
 	}
+	if (IsA(node, ArrayCoerceExpr))
+	{
+		ArrayCoerceExpr *expr = (ArrayCoerceExpr *) node;
+		Expr	   *arg;
+		ArrayCoerceExpr *newexpr;
+
+		/*
+		 * Reduce constants in the ArrayCoerceExpr's argument, then build
+		 * a new ArrayCoerceExpr.
+		 */
+		arg = (Expr *) eval_const_expressions_mutator((Node *) expr->arg,
+													  context);
+
+		newexpr = makeNode(ArrayCoerceExpr);
+		newexpr->arg = arg;
+		newexpr->elemfuncid = expr->elemfuncid;
+		newexpr->resulttype = expr->resulttype;
+		newexpr->resulttypmod = expr->resulttypmod;
+		newexpr->isExplicit = expr->isExplicit;
+		newexpr->coerceformat = expr->coerceformat;
+
+		/*
+		 * If constant argument and it's a binary-coercible or immutable
+		 * conversion, we can simplify it to a constant.
+		 */
+		if (arg && IsA(arg, Const) &&
+			(!OidIsValid(newexpr->elemfuncid) ||
+			 func_volatile(newexpr->elemfuncid) == PROVOLATILE_IMMUTABLE))
+			return (Node *) evaluate_expr((Expr *) newexpr,
+										  newexpr->resulttype,
+										  newexpr->resulttypmod);
+
+		/* Else we must return the partially-simplified node */
+		return (Node *) newexpr;
+	}
 	if (IsA(node, CaseExpr))
 	{
 		/*----------