Hi I believe this should work
<div class="col-xs-8">
<?php $count = 1; ?>
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<?php if $count = 1; ?>
<!-- First post Wrapper-->
<div class="first-post">
<article class="post1"></article>
</div>
<!-- Rest all post Wrapper-->
<div class="all-posts">
<?php else : ?>
<article class="post<?php echo $count; ?>"></article>
<?php endif; ?>
<?php $count++; endwhile; endif; ?>
</div>
</div>