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
```
/** For explicitly passed size of pool and number of used tasks.
      * This method could be used to calculate threshold depending on number of tasks in replication queue.
      */
    UInt64 getMaxSourcePartsSizeForMerge(size_t pool_size, size_t pool_used) const;
    /** Get maximum total size of parts to do mutation, at current moment of time.
      * It depends only on amount of free space in disk.
      */
    UInt64 getMaxSourcePartSizeForMutation() const;
    /** Selects which parts to merge. Uses a lot of heuristics.
      *
      * can_merge - a function that determines if it is possible to merge a pair of adjacent parts.
      *  This function must coordinate merge with inserts and other merges, ensuring that
      *  - Parts between which another part can still appear can not be merged. Refer to METR-7001.
      *  - A part that already merges with something in one place, you can not start to merge into something else in another place.
      */
    SelectPartsDecision selectPartsToMerge(
        FutureMergedMutatedPart & future_part,
        bool aggressive,
        size_t max_total_size_to_merge,
        const AllowedMergingPredicate & can_merge,
        bool merge_with_ttl_allowed,
        String * out_disable_reason = nullptr);
    /** Select all the parts in the specified partition for merge, if possible.
      * final - choose to merge even a single part - that is, allow to merge one part "with itself",
      * but if setting optimize_skip_merged_partitions is true than single part with level > 0
      * and without expired TTL won't be merged with itself.
      */
    SelectPartsDecision selectAllPartsToMergeWithinPartition(
        FutureMergedMutatedPart & future_part,
        UInt64 & available_disk_space,