[7.x] Add 'loadMorph' and 'loadMorphCount' on Eloquent Model

[gdebrauwer]

The loadMorph method already exists on Eloquent collection since v5.6 (#23626).
I recently added the loadMorphCount method to the Eloquent collection (#32739).

If you have a collection of Eloquent models, these methods work very well. But what if you only have one model? You have to write this kind of if-statements:

$comment = Comment::first();

if ($comment->commentable instanceof Post) {
    $comment->commentable->load('likes');
    // or
    $comment->commentable->loadCount('likes');
} elseif ($comment->commentable instance Video) {
    $comment->commentable->load('views');
    // or
    $comment->commentable->loadCount('views');
}

By adding loadMorph and loadMorphCount methods to the Eloquent Model, this becomes a lot more elegant:

$comment = Comment::first()

$comment->loadMorph('commentable', [
   Post::class => ['likes'],
   Video::class => ['views'],
]);

$comment->loadMorphCount('commentable', [
   Post::class => ['likes'],
   Video::class => ['views'],
]);