Fix wrong results of Left Anti Semi (Not-In) Join

[avamingli]

This pr has 2 commits, please review by commits.

See more details and discussion in https://github.com/greenplum-db/gpdb/pull/15663, https://github.com/greenplum-db/gpdb/issues/15662.

Most codes are same with some refinement & conflicts resolved for CBDB.

Fix wrong results of Left Anti Semi (Not-In) Join

CBDB will try to convert a NOT IN sql into a Left Anti Semi (Not-In) Join
by using cdb_find_nonnullable_vars_walker() to find if there might be nullable
values from inner or outer side.

If there is NullTest, expression_tree_walker will iterate the tree using
(NullTest*)Node->arg.
Example:

select c1 from t1_lasj where c1 not in (
	select c1n from t2_lasj_has_null where c1n is null or c1n is null)
	and c1 is not null;

Expression c1n is null would be like:
NullTest [nulltesttype=IS_NULL argisrow=false location=102]
[arg] Var [varno=1 varattno=1 vartype=23 varnoold=1 varoattno=1]

Recursive cdb_find_nonnullable_vars_walker will first check the node->arg Var and
insert into nonNullableVars.
That's incorrect for NullTest type IS_NULL.
We should consifer the NullTest under OR expression and recursive OR expression.
Add a field nullableVars to indentify the vars might be nullable, and must be
eliminated from nonNullableVars finally.
This is more strict, but ensure right results at least.

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 | <>

closes: #50

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[kongfanshen-0801]

LGTM!