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
```
if (command.type == MutationCommand::DROP_COLUMN)
            removed_columns.insert(command.column_name);
        if (command.type == MutationCommand::RENAME_COLUMN)
            renamed_columns_to_from.emplace(command.rename_to, command.column_name);
    }
    Names source_column_names = source_part->getColumns().getNames();
    NameSet source_columns_name_set(source_column_names.begin(), source_column_names.end());
    for (auto it = storage_columns.begin(); it != storage_columns.end();)
    {
        if (updated_header.has(it->name))
        {
            auto updated_type = updated_header.getByName(it->name).type;
            if (updated_type != it->type)
                it->type = updated_type;
            ++it;
        }
        else
        {
            if (!source_columns_name_set.count(it->name))
            {
                
                
                auto renamed_it = renamed_columns_to_from.find(it->name);
                if (renamed_it != renamed_columns_to_from.end()
                    && source_columns_name_set.count(renamed_it->second))
                    ++it;
                else
                    it = storage_columns.erase(it);
            }
            else