skip user join on assignment override invisible users query

test plan:
* specs

closes #LS-1419

Change-Id: Ie82488b3885f9c8e5f2c029de003398bba574379
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/246779
Reviewed-by: Keith Garner <kgarner@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

lib/api/v1/assignment_override.rb

@@ -381,7 +381,7 @@ def update_assignment_overrides(overrides, overrides_data, updating_user: nil)
   def invisible_users_and_overrides_for_user(context, user, existing_overrides)
     # get the student overrides the user can't see and ensure those overrides are included
     visible_user_ids = context.enrollments_visible_to(user).select(:user_id)
-    invisible_user_ids = context.users.where.not(id: visible_user_ids).pluck(:id)
+    invisible_user_ids = context.enrollments.where.not(:user_id => visible_user_ids).distinct.pluck(:user_id)
     invisible_override_ids = existing_overrides.select{ |ov|
       ov.set_type == 'ADHOC' &&
       !ov.visible_student_overrides(visible_user_ids)