sql: increase row_count when adding CHECK constraint

If CHECK constraint is added using ALTER TABLE statement, row_count
should be incremented. Note that row_count does not increase if CHECK
constraint is created during the execution of CREATE TABLE statement.
For example:

box.execute('CREATE TABLE t1(id INTEGER PRIMARY KEY);')
box.execute('ALTER TABLE t1 ADD CONSTRAINT ck1 CHECK(id > 0);')

Should return:
- row_count: 1

However, prior to this patch it was always equal to zero. Let's fix
this passing appropriate flag to OP_SInsert opcode.

Closes #4363