version 21.5.5.12 (official build)

I'm trying to change data type for a column that is used in the TTL expression.
It works well when the table is empty.
When the table is not empty, the mutation fails.
As a result the table ends up in an inconsistent state.

```
create table test
(
InsertionDateTime DateTime,
TTLDays Int32 DEFAULT CAST(365, 'Int32')
)
Engine=MergeTree()
order by tuple()
TTL InsertionDateTime + toIntervalDay(TTLDays);

Ok.

insert into test values (now(), 23);

Ok.

ALTER TABLE test modify column TTLDays Int16 DEFAULT CAST(365, 'Int16');

Code: 341, e.displayText() = DB::Exception: Exception happened during execution of mutation 'mutation_2.txt' with part 'all_1_1_0' reason: 'Code: 10, e.displayText() = DB::Exception: Not found column InsertionDateTime in block. There are only columns: TTLDays (version 21.5.5.12 (official build))'. This error maybe retryable or not. In case of unretryable error, mutation can be killed with KILL MUTATION query (version 21.5.5.12 (official build)) [DB Errorcode=341]

insert into test values (now(), 23);

Ok.

select table, column, type, sum(rows) rows, sum(column_bytes_on_disk) on_disk
from system.parts_columns where column like 'TTLDays'
group by database, table, column, type

table | column  | type  | rows | on_disk
------+---------+-------+------+--------
test  | TTLDays | Int32 |    1 |      86
test  | TTLDays | Int16 |    1 |      84
```
actions_chain.addStep();
            for (const auto & kv : stage.column_to_updated)
                stage.analyzer->appendExpression(actions_chain, kv.second, dry_run);
            auto & actions = actions_chain.getLastStep().actions();
            for (const auto & kv : stage.column_to_updated)
            {
                auto column_name = kv.second->getColumnName();
                const auto & dag_node = actions->findInIndex(column_name);
                const auto & alias = actions->addAlias(dag_node, kv.first);
                actions->addOrReplaceInIndex(alias);
            }
        }
        
        actions_chain.addStep();
        actions_chain.getLastStep().required_output.clear();
        ActionsDAG::NodeRawConstPtrs new_index;
        for (const auto & name : stage.output_columns)
            actions_chain.getLastStep().addRequiredOutput(name);
        actions_chain.getLastActions();
        actions_chain.finalize();
        
        for (const auto & column : actions_chain.steps.front()->getRequiredColumns())
            prepared_stages[i - 1].output_columns.insert(column.name);
    }
    
    auto select = std::make_shared<ASTSelectQuery>();
    select->setExpression(ASTSelectQuery::Expression::SELECT, std::make_shared<ASTExpressionList>());
    for (const auto & column_name : prepared_stages[0].output_columns)
        select->select()->children.push_back(std::make_shared<ASTIdentifier>(column_name));