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
```
parts_ranges.back().emplace_back(part_info);
        
        if (prev_part && part->info.partition_id == (*prev_part)->info.partition_id
            && part->info.min_block <= (*prev_part)->info.max_block)
        {
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Part {} intersects previous part {}", part->name, (*prev_part)->name);
        }
        prev_part = &part;
    }
    if (parts_selected_precondition == 0)
    {
        if (out_disable_reason)
            *out_disable_reason = "No parts satisfy preconditions for merge";
        return SelectPartsDecision::CANNOT_SELECT;
    }
    IMergeSelector::PartsRange parts_to_merge;
    if (metadata_snapshot->hasAnyTTL() && merge_with_ttl_allowed && !ttl_merges_blocker.isCancelled())
    {
        
        TTLDeleteMergeSelector delete_ttl_selector(
                next_delete_ttl_merge_times_by_partition,
                current_time,
                data_settings->merge_with_ttl_timeout,
                data_settings->ttl_only_drop_parts);
        parts_to_merge = delete_ttl_selector.select(parts_ranges, max_total_size_to_merge);
        if (!parts_to_merge.empty())
        {
            future_part.merge_type = MergeType::TTL_DELETE;
        }
        else if (metadata_snapshot->hasAnyRecompressionTTL())