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
```
{
    return ((settings.min_rows_to_fsync_after_merge && input_rows >= settings.min_rows_to_fsync_after_merge)
        || (settings.min_compressed_bytes_to_fsync_after_merge && input_bytes >= settings.min_compressed_bytes_to_fsync_after_merge));
}
MergeTreeData::MutableDataPartPtr MergeTreeDataMergerMutator::mergePartsToTemporaryPart(
    const FutureMergedMutatedPart & future_part,
    const StorageMetadataPtr & metadata_snapshot,
    MergeList::Entry & merge_entry,
    TableLockHolder & holder,
    time_t time_of_merge,
    ContextPtr context,
    const ReservationPtr & space_reservation,
    bool deduplicate,
    const Names & deduplicate_by_columns,
    const MergeTreeData::MergingParams & merging_params,
    const IMergeTreeDataPart * parent_part,
    const String & prefix)
{
    const String tmp_prefix = parent_part ? prefix : "tmp_merge_";
    if (merges_blocker.isCancelled())
        throw Exception("Cancelled merging parts", ErrorCodes::ABORTED);
    
    
    if (isTTLMergeType(future_part.merge_type) && ttl_merges_blocker.isCancelled())
        throw Exception("Cancelled merging parts with TTL", ErrorCodes::ABORTED);
    const MergeTreeData::DataPartsVector & parts = future_part.parts;
    LOG_DEBUG(log, "Merging {} parts: from {} to {} into {}", parts.size(), parts.front()->name, parts.back()->name, future_part.type.toString());
    if (deduplicate)
    {
        if (deduplicate_by_columns.empty())