PERF: Recover `changes_applied` performance (#31698)

#30985 caused `object.save` performance regression since calling
`changes` in `changes_applied` is very slow.
We don't need to call the expensive method in `changes_applied` as long
as `@attributes` is tracked by mutation tracker.

https://gist.github.com/kamipo/1a9f4f3891803b914fc72ede98268aa2

Before:

```
Warming up --------------------------------------
create_string_columns
                        73.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        722.256  (± 5.8%) i/s -      3.650k in   5.073031s
```

After:

```
Warming up --------------------------------------
create_string_columns
                        96.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        950.224  (± 7.7%) i/s -      4.800k in   5.084837s
```

2 files changed, 2 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 3de6fe566d..df4c79b0f6 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -32,9 +32,7 @@ module ActiveRecord
       # <tt>reload</tt> the record and clears changed attributes.
       def reload(*)
         super.tap do
-          @previously_changed = ActiveSupport::HashWithIndifferentAccess.new
           @mutations_before_last_save = nil
-          @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
           @mutations_from_database = nil
         end
       end
@@ -114,12 +112,12 @@ module ActiveRecord
 
       # Alias for +changed+
       def changed_attribute_names_to_save
-        changes_to_save.keys
+        mutations_from_database.changed_attribute_names
       end
 
       # Alias for +changed_attributes+
       def attributes_in_database
-        changes_to_save.transform_values(&:first)
+        mutations_from_database.changed_values
       end
 
       private
diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index c1b1a5334a..cdd54cc502 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -362,7 +362,6 @@ module ActiveRecord
       became = klass.new
       became.instance_variable_set("@attributes", @attributes)
       became.instance_variable_set("@mutations_from_database", @mutations_from_database) if defined?(@mutations_from_database)
-      became.instance_variable_set("@changed_attributes", attributes_changed_by_setter)
       became.instance_variable_set("@new_record", new_record?)
       became.instance_variable_set("@destroyed", destroyed?)
       became.errors.copy!(errors)