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
```
{
            TTLRecompressMergeSelector recompress_ttl_selector(
                    next_recompress_ttl_merge_times_by_partition,
                    current_time,
                    data_settings->merge_with_recompression_ttl_timeout,
                    metadata_snapshot->getRecompressionTTLs());
            parts_to_merge = recompress_ttl_selector.select(parts_ranges, max_total_size_to_merge);
            if (!parts_to_merge.empty())
                future_part.merge_type = MergeType::TTL_RECOMPRESS;
        }
    }
    if (parts_to_merge.empty())
    {
        SimpleMergeSelector::Settings merge_settings;
        
        merge_settings.max_parts_to_merge_at_once = data_settings->max_parts_to_merge_at_once;
        if (aggressive)
            merge_settings.base = 1;
        parts_to_merge = SimpleMergeSelector(merge_settings)
                            .select(parts_ranges, max_total_size_to_merge);
        
        if (parts_to_merge.size() == 1)
            throw Exception("Logical error: merge selector returned only one part to merge", ErrorCodes::LOGICAL_ERROR);
        if (parts_to_merge.empty())
        {
            if (out_disable_reason)
                *out_disable_reason = "There is no need to merge parts according to merge selector algorithm";
            return SelectPartsDecision::CANNOT_SELECT;
        }
    }