Correct comments in convert_IN_to_antijoin()

Incorrect example comments:
The transformation is to rewrite a query of the form:
	select c1 from t1 where c1 NOT IN (select c2 from t2);
			(to)
	select c1 from t1 left anti semi join (select 0 as zero, c2 from t2) foo
		ON (c1 = c2) IS NOT FALSE where zero is NULL;
Correct it to:
The transformation is to rewrite a query of the form:
	select c1 from t1 where c1 NOT IN (select c2 from t2);
			(to)
	select c1 from t1 left anti semi join (select 0 as zero, c2 from t2) foo
		ON (c1 != c2) IS NOT FALSE where zero is NULL;

SQL NOT IN should be converted to Left Anti Semi (not-in) Join
with join condition c1 != c2.

Any non-null values from t1 don't match values from t2 shoule be kept and
IS NOT FALSE will return TRUE.

GDB checks after function make_lasj_quals()
join_expr->quals:
BoolExpr [boolop=NOT_EXPR]
        OpExpr [opno=518 opfuncid=144 opresulttype=16 opretset=false]
                Var [varno=1 varattno=1 vartype=23 varnoold=1 varoattno=1]
                Var [varno=2 varattno=1 vartype=23 varnoold=2 varoattno=1]

And oid 518 is a '<>' operator in pg_operator.
select oprname from pg_operator where oid = 518;
-[ RECORD 1 ]
oprname | <>

Authored-by: Zhang Mingli avamingli@gmail.com

Author: avamingli

src/backend/cdb/cdbsubselect.c

@@ -1455,7 +1455,7 @@ is_exprs_nullable_internal(Node *exprs, List *nonnullable_vars)
  *		select c1 from t1 where c1 NOT IN (select c2 from t2);
  *						(to)
  *		select c1 from t1 left anti semi join (select 0 as zero, c2 from t2) foo
- *						  ON (c1 = c2) IS NOT FALSE where zero is NULL;
+ *						  ON (c1 != c2) IS NOT FALSE where zero is NULL;
  *
  * The pseudoconstant column zero is needed to correctly pipe in the NULLs
  * from the subselect upstream.