Fix grammar's AND/OR flattening to work with operator_precedence_warning.

It'd be good for "(x AND y) AND z" to produce a three-child AND node
whether or not operator_precedence_warning is on, but that failed to
happen when it's on because makeAndExpr() didn't look through the added
AEXPR_PAREN node.  This has no effect on generated plans because prepqual.c
would flatten the AND nest anyway; but it does affect the number of parens
printed in ruleutils.c, for example.  I'd already fixed some similar
hazards in parse_expr.c in commit abb16465, but didn't think to search
gram.y for problems of this ilk.  Per gripe from Jean-Pierre Pelletier.

Report: <fa0535ec6d6428cfec40c7e8a6d11156@mail.gmail.com>

src/backend/parser/gram.y

@@ -14643,10 +14643,16 @@ doNegateFloat(Value *v)
 static Node *
 makeAndExpr(Node *lexpr, Node *rexpr, int location)
 {
+	Node	   *lexp = lexpr;
+
+	/* Look through AEXPR_PAREN nodes so they don't affect flattening */
+	while (IsA(lexp, A_Expr) &&
+		   ((A_Expr *) lexp)->kind == AEXPR_PAREN)
+		lexp = ((A_Expr *) lexp)->lexpr;
 	/* Flatten "a AND b AND c ..." to a single BoolExpr on sight */
-	if (IsA(lexpr, BoolExpr))
+	if (IsA(lexp, BoolExpr))
 	{
-		BoolExpr *blexpr = (BoolExpr *) lexpr;
+		BoolExpr *blexpr = (BoolExpr *) lexp;
 
 		if (blexpr->boolop == AND_EXPR)
 		{
@@ -14660,10 +14666,16 @@ makeAndExpr(Node *lexpr, Node *rexpr, int location)
 static Node *
 makeOrExpr(Node *lexpr, Node *rexpr, int location)
 {
+	Node	   *lexp = lexpr;
+
+	/* Look through AEXPR_PAREN nodes so they don't affect flattening */
+	while (IsA(lexp, A_Expr) &&
+		   ((A_Expr *) lexp)->kind == AEXPR_PAREN)
+		lexp = ((A_Expr *) lexp)->lexpr;
 	/* Flatten "a OR b OR c ..." to a single BoolExpr on sight */
-	if (IsA(lexpr, BoolExpr))
+	if (IsA(lexp, BoolExpr))
 	{
-		BoolExpr *blexpr = (BoolExpr *) lexpr;
+		BoolExpr *blexpr = (BoolExpr *) lexp;
 
 		if (blexpr->boolop == OR_EXPR)
 		{