[ORCA] optimize eliminate self comparison

'PexprEliminateSelfComparison' only uses the 'pcrsNotNull'
from the topmost expression to filter the nullable columns.
This can lead the PexprEliminateSelfComparison cannot apply
to the subquery properly.

create table t1(a int not null, b int not null);
create table t2(like t1);
create table t3(like t1);
select * from t2  left join (select  t2.a , t2.b  from t1, t2 where t1.a
	< t1.a) as t on t2. a = t.a;

the plan for it from orca is
 Gather Motion 3:1  (slice1; segments: 3)
   ->  Hash Left Join
         Hash Cond: (t2.a = t2_1.a)
         ->  Seq Scan on t2
         ->  Hash
               ->  Nested Loop
                     Join Filter: true
                     ->  Seq Scan on t2 t2_1
                     ->  Materialize
                           ->  Broadcast Motion 3:3  (slice2; segments: 3)
                                 ->  Seq Scan on t1
                                       Filter: (a < a)
the self comparison in subquery is not eliminated.

This commit is to optimize it by fetching 'pcrsNotNull' from
the current logical expression and apply them to its child
scalar expression.

src/backend/gporca/libgpopt/src/operators/CExpressionPreprocessor.cpp

@@ -77,12 +77,17 @@ CExpressionPreprocessor::PexprEliminateSelfComparison(CMemoryPool *mp,
 	GPOS_CHECK_STACK_SIZE;
 	GPOS_ASSERT(nullptr != mp);
 	GPOS_ASSERT(nullptr != pexpr);
+	COperator *pop = pexpr->Pop();
 
 	if (CUtils::FScalarCmp(pexpr))
 	{
 		return CPredicateUtils::PexprEliminateSelfComparison(mp, pexpr,
 															 pcrsNotNull);
 	}
+	if (pop->FLogical())
+	{
+		pcrsNotNull = pexpr->DeriveNotNullColumns();
+	}
 
 	// recursively process children
 	const ULONG arity = pexpr->Arity();
@@ -94,7 +99,6 @@ CExpressionPreprocessor::PexprEliminateSelfComparison(CMemoryPool *mp,
 		pdrgpexprChildren->Append(pexprChild);
 	}
 
-	COperator *pop = pexpr->Pop();
 	pop->AddRef();
 
 	return GPOS_NEW(mp) CExpression(mp, pop, pdrgpexprChildren);