Fix incorrect check in PciIntervalFromScalarIDF()

When using CConstraintInterval to derive partition filters, we cannot
use a casted Ident, since the cast information is lost. This tripped an
assertion. Also implement FIdentIDFConst().

libgpopt/include/gpopt/operators/CPredicateUtils.h

@@ -209,6 +209,10 @@ namespace gpopt
 			static
 			BOOL FCompareIdentToConst(CExpression *pexpr);
 
+			// is the given expression of the form (col IS DISTINCT FROM const)
+			static
+			BOOL FIdentIDFConst(CExpression *pexpr);
+
 			// checks if comparison is between two columns, or a column and a const
 			static
 			BOOL FCompareColToConstOrCol(CExpression *pexprScalar);

libgpopt/src/base/CConstraintInterval.cpp

@@ -445,7 +445,7 @@ CConstraintInterval::PciIntervalFromScalarIDF
 	GPOS_ASSERT(NULL != pexpr);
 	GPOS_ASSERT(CPredicateUtils::FIDF(pexpr));
 
-	if (CPredicateUtils::FIdentIDFConstIgnoreCast(pexpr))
+	if (CPredicateUtils::FIdentIDFConst(pexpr))
 	{
 		// column
 #ifdef GPOS_DEBUG

libgpopt/src/operators/CPredicateUtils.cpp

@@ -1086,6 +1086,39 @@ CPredicateUtils::FCompareIdentToConst
 	return true;
 }
 
+// is the given expression of the form (col IS DISTINCT FROM const)
+BOOL
+CPredicateUtils::FIdentIDFConst
+	(
+	CExpression *pexpr
+	)
+{
+	COperator *pop = pexpr->Pop();
+
+	if (COperator::EopScalarIsDistinctFrom != pop->Eopid())
+	{
+		return false;
+	}
+
+	CExpression *pexprLeft = (*pexpr)[0];
+	CExpression *pexprRight = (*pexpr)[1];
+
+	// left side must be scalar ident
+	if (COperator::EopScalarIdent != pexprLeft->Pop()->Eopid())
+	{
+		return false;
+	}
+
+	// right side must be a constant
+	if (COperator::EopScalarConst != pexprRight->Pop()->Eopid())
+	{
+		return false;
+	}
+
+	return true;
+}
+
+
 // is the given expression is of the form (col IS DISTINCT FROM const)
 // ignoring cast on either sides
 BOOL