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
```
private:
    ASTPtr prepare(bool dry_run);
    struct Stage;
    ASTPtr prepareInterpreterSelectQuery(std::vector<Stage> &prepared_stages, bool dry_run);
    QueryPipelinePtr addStreamsForLaterStages(const std::vector<Stage> & prepared_stages, QueryPlan & plan) const;
    std::optional<SortDescription> getStorageSortDescriptionIfPossible(const Block & header) const;
    ASTPtr getPartitionAndPredicateExpressionForMutationCommand(const MutationCommand & command) const;
    StoragePtr storage;
    StorageMetadataPtr metadata_snapshot;
    MutationCommands commands;
    ContextPtr context;
    bool can_execute;
    SelectQueryOptions select_limits;
    ASTPtr mutation_ast;
    
    
    std::unique_ptr<InterpreterSelectQuery> select_interpreter;