Avoid duplicating complex expression in comparisons

When comparing a nullable expression to a non-nullable one, a  `NULL` result  always
represent a difference.

This makes it possible to avoid duplicating the nullable expression by mapping
the `NULL` result to a `FALSE` (when comparing for equality).

Fixes dotnet#34165.

src/EFCore.Relational/Query/SqlNullabilityProcessor.cs

@@ -1815,10 +1815,23 @@ private SqlExpression RewriteNullSemantics(
         // doing a full null semantics rewrite - removing all nulls from truth table
         nullable = false;
 
-        // (a == b && (a != null && b != null)) || (a == null && b == null)
-        body = _sqlExpressionFactory.OrElse(
-            _sqlExpressionFactory.AndAlso(body, _sqlExpressionFactory.AndAlso(leftIsNotNull, rightIsNotNull)),
-            _sqlExpressionFactory.AndAlso(leftIsNull, rightIsNull));
+        if (leftNullable && rightNullable
+            || leftIsNull is SqlUnaryExpression { Operand: ColumnExpression }
+            || rightIsNull is SqlUnaryExpression { Operand: ColumnExpression })
+        {
+            // (a == b && (a != null && b != null)) || (a == null && b == null)
+            body = _sqlExpressionFactory.OrElse(
+                _sqlExpressionFactory.AndAlso(body, _sqlExpressionFactory.AndAlso(leftIsNotNull, rightIsNotNull)),
+                _sqlExpressionFactory.AndAlso(leftIsNull, rightIsNull));
+        }
+        else
+        {
+            // a == b -> CASE WHEN a == b THEN TRUE ELSE FALSE END
+            body = _sqlExpressionFactory.Case(
+                [new(body, _sqlExpressionFactory.Constant(true, body.Type, body.TypeMapping))],
+                _sqlExpressionFactory.Constant(false, body.Type, body.TypeMapping));
+        }
+
 
         if (sqlBinaryExpression.OperatorType == ExpressionType.NotEqual)
         {