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
```
MergeTreeData::DataPartsVector parts;
    parts.reserve(parts_to_merge.size());
    for (IMergeSelector::Part & part_info : parts_to_merge)
    {
        const MergeTreeData::DataPartPtr & part = *static_cast<const MergeTreeData::DataPartPtr *>(part_info.data);
        parts.push_back(part);
    }
    LOG_DEBUG(log, "Selected {} parts from {} to {}", parts.size(), parts.front()->name, parts.back()->name);
    future_part.assign(std::move(parts));
    return SelectPartsDecision::SELECTED;
}
SelectPartsDecision MergeTreeDataMergerMutator::selectAllPartsToMergeWithinPartition(
    FutureMergedMutatedPart & future_part,
    UInt64 & available_disk_space,
    const AllowedMergingPredicate & can_merge,
    const String & partition_id,
    bool final,
    const StorageMetadataPtr & metadata_snapshot,
    String * out_disable_reason,
    bool optimize_skip_merged_partitions)
{
    MergeTreeData::DataPartsVector parts = selectAllPartsFromPartition(partition_id);
    if (parts.empty())
        return SelectPartsDecision::CANNOT_SELECT;
    if (!final && parts.size() == 1)
    {
        if (out_disable_reason)
            *out_disable_reason = "There is only one part inside partition";
        return SelectPartsDecision::CANNOT_SELECT;
    }