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
```
if (future_part.parts.size() != 1)
        throw Exception("Trying to mutate " + toString(future_part.parts.size()) + " parts, not one. "
            "This is a bug.", ErrorCodes::LOGICAL_ERROR);
    CurrentMetrics::Increment num_mutations{CurrentMetrics::PartMutation};
    const auto & source_part = future_part.parts[0];
    auto storage_from_source_part = StorageFromMergeTreeDataPart::create(source_part);
    auto context_for_reading = Context::createCopy(context);
    context_for_reading->setSetting("max_streams_to_max_threads_ratio", 1);
    context_for_reading->setSetting("max_threads", 1);
    
    context_for_reading->setSetting("force_index_by_date", Field(0));
    context_for_reading->setSetting("force_primary_key", Field(0));
    MutationCommands commands_for_part;
    for (const auto & command : commands)
    {
        if (command.partition == nullptr || future_part.parts[0]->info.partition_id == data.getPartitionIDFromQuery(
                command.partition, context_for_reading))
            commands_for_part.emplace_back(command);
    }
    if (source_part->isStoredOnDisk() && !isStorageTouchedByMutations(
        storage_from_source_part, metadata_snapshot, commands_for_part, Context::createCopy(context_for_reading)))
    {
        LOG_TRACE(log, "Part {} doesn't change up to mutation version {}", source_part->name, future_part.part_info.mutation);
        return data.cloneAndLoadDataPartOnSameDisk(source_part, "tmp_clone_", future_part.part_info, metadata_snapshot);
    }
    else
    {
        LOG_TRACE(log, "Mutating part {} to mutation version {}", source_part->name, future_part.part_info.mutation);
    }
    BlockInputStreamPtr in = nullptr;