ddl: skip reorg for `MODIFY COLUMN` in some cases

## Enhancement

Currently, we have a strict criterion to determine whether to do `MODIFY COLUMN` with reorg.

For example, `VARCHAR` to `CHAR` need reorg, since it may has truncation, as `VARCHAR` doesn't trim trailling space.

But in some scenarios, the reorg can be skipped. For example, if we want to modify `BIGINT` to `INT` with **strict SQL mode** (important), the reorg is redundant:
1. If there are some out-of-range values, the DDL should fail.
2. If all the values are in the range of `INT`, we don't need to rewrite the data as the encoding is not changed.

So in such cases, we can also do the modification without changing data too.