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
```
const AllowedMergingPredicate & can_merge,
        const String & partition_id,
        bool final,
        const StorageMetadataPtr & metadata_snapshot,
        String * out_disable_reason = nullptr,
        bool optimize_skip_merged_partitions = false);
    /** Merge the parts.
      * If `reservation != nullptr`, now and then reduces the size of the reserved space
      *  is approximately proportional to the amount of data already written.
      *
      * Creates and returns a temporary part.
      * To end the merge, call the function renameMergedTemporaryPart.
      *
      * time_of_merge - the time when the merge was assigned.
      * Important when using ReplicatedGraphiteMergeTree to provide the same merge on replicas.
      */
    MergeTreeData::MutableDataPartPtr mergePartsToTemporaryPart(
        const FutureMergedMutatedPart & future_part,
        const StorageMetadataPtr & metadata_snapshot,
        MergeListEntry & merge_entry,
        TableLockHolder & table_lock_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 = nullptr,
        const String & prefix = "");