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
```
merging_columns = storage_columns;
        merging_column_names = all_column_names;
        gathering_columns.clear();
        gathering_column_names.clear();
    }
    /** Read from all parts, merge and write into a new one.
      * In passing, we calculate expression for sorting.
      */
    Pipes pipes;
    UInt64 watch_prev_elapsed = 0;
    
    
    bool read_with_direct_io = false;
    if (data_settings->min_merge_bytes_to_use_direct_io != 0)
    {
        size_t total_size = 0;
        for (const auto & part : parts)
        {
            total_size += part->getBytesOnDisk();
            if (total_size >= data_settings->min_merge_bytes_to_use_direct_io)
            {
                LOG_DEBUG(log, "Will merge parts reading files in O_DIRECT");
                read_with_direct_io = true;
                break;
            }
        }
    }
    MergeStageProgress horizontal_stage_progress(
        column_sizes ? column_sizes->keyColumnsWeight() : 1.0);
    for (const auto & part : parts)