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
```
const NamesAndTypesList & storage_columns,
    const ExpressionActionsPtr & sorting_key_expr,
    const IndicesDescription & indexes,
    const ProjectionsDescription & projections,
    const MergeTreeData::MergingParams & merging_params,
    NamesAndTypesList & gathering_columns, Names & gathering_column_names,
    NamesAndTypesList & merging_columns, Names & merging_column_names)
{
    Names sort_key_columns_vec = sorting_key_expr->getRequiredColumns();
    std::set<String> key_columns(sort_key_columns_vec.cbegin(), sort_key_columns_vec.cend());
    for (const auto & index : indexes)
    {
        Names index_columns_vec = index.expression->getRequiredColumns();
        std::copy(index_columns_vec.cbegin(), index_columns_vec.cend(),
                  std::inserter(key_columns, key_columns.end()));
    }
    for (const auto & projection : projections)
    {
        Names projection_columns_vec = projection.required_columns;
        std::copy(projection_columns_vec.cbegin(), projection_columns_vec.cend(),