MDEV-12836 Avoid table rebuild when removing of auto_increment settings

Field::is_equal(): treat old type and new one without AUTO_INCREMENT as equal

Closes #1208

mysql-test/suite/innodb/r/instant_auto_inc.result

@@ -0,0 +1,21 @@
+create table t(id int primary key, a int) engine=InnoDB;
+insert into t (id, a) values (1, 1);
+alter table t modify column id int auto_increment;
+check table t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+insert into t (a) values (2);
+alter table t modify column id int, algorithm=instant;
+check table t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+insert into t (id, a) values (3, 3);
+select * from t;
+id	a
+1	1
+2	2
+3	3
+check table t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+drop table t;

mysql-test/suite/innodb/t/instant_auto_inc.test

@@ -0,0 +1,13 @@
+--source include/have_innodb.inc
+
+create table t(id int primary key, a int) engine=InnoDB;
+insert into t (id, a) values (1, 1);
+alter table t modify column id int auto_increment;
+check table t;
+insert into t (a) values (2);
+alter table t modify column id int, algorithm=instant;
+check table t;
+insert into t (id, a) values (3, 3);
+select * from t;
+check table t;
+drop table t;

sql/field.cc

@@ -3469,7 +3469,7 @@ uint Field_new_decimal::is_equal(Create_field *new_field)
   return ((new_field->type_handler() == type_handler()) &&
           ((new_field->flags & UNSIGNED_FLAG) == 
            (uint) (flags & UNSIGNED_FLAG)) &&
-          ((new_field->flags & AUTO_INCREMENT_FLAG) ==
+          ((new_field->flags & AUTO_INCREMENT_FLAG) <=
            (uint) (flags & AUTO_INCREMENT_FLAG)) &&
           (new_field->length == max_display_length()) &&
           (new_field->decimals == dec));
@@ -9535,7 +9535,8 @@ bool Field_num::eq_def(const Field *field) const
 
 uint Field_num::is_equal(Create_field *new_field)
 {
-  if ((new_field->flags ^ flags) & (UNSIGNED_FLAG | AUTO_INCREMENT_FLAG))
+  if (((new_field->flags & UNSIGNED_FLAG) != (flags & UNSIGNED_FLAG)) ||
+      ((new_field->flags & AUTO_INCREMENT_FLAG) > (flags & AUTO_INCREMENT_FLAG)))
     return IS_EQUAL_NO;
 
   const Type_handler *th= type_handler(), *new_th = new_field->type_handler();