Port _filter_autop() changes from D8 to D7

When you said

The _filter_autop() function was updated to support HTML5 block tags a long time ago

you didn't include a link to the issue containing the change that needs backporting.

I was able to trace that change to #900956: Update autop filter to include missing block level tags, committed in 2013:

commit e5313544bce78b8aef5ed2fbd79f08a4d6aff5dc
Author: Dries <dries@buytaert.net>
Date:   Fri Mar 8 09:12:35 2013 -0500

    Issue #900956 by quicksketch, sun: update autop filter to include missing block level tags.

diff --git a/core/modules/filter/filter.module b/core/modules/filter/filter.module
index b827b4fcb1..725d9312b7 100644
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -1688,7 +1688,7 @@ function _filter_htmlcorrector($text) {
  */
 function _filter_autop($text) {
   // All block level tags
-  $block = '(?:table|thead|tfoot|caption|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|select|form|blockquote|address|p|h[1-6]|hr|article|aside|details|figcaption|figure|footer|header|hgroup|menu|nav|section|summary)';
+  $block = '(?:table|thead|tfoot|caption|col|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|select|option|form|map|area|blockquote|address|math|input|p|h[1-6]|fieldset|legend|hr|article|aside|details|figcaption|figure|footer|header|hgroup|menu|nav|section|summary)';
 
   // Split at opening and closing PRE, SCRIPT, STYLE, OBJECT, IFRAME tags
   // and comments. We don't apply any processing to the contents of these tags
@@ -1740,7 +1740,7 @@ function _filter_autop($text) {
       $chunk = preg_replace('!(</?' . $block . '[^>]*>)\s*</p>!', "$1", $chunk);
       $chunk = preg_replace('|(?<!<br />)\s*\n|', "<br />\n", $chunk); // make line breaks
       $chunk = preg_replace('!(</?' . $block . '[^>]*>)\s*<br />!', "$1", $chunk);
-      $chunk = preg_replace('!<br />(\s*</?(?:p|li|div|th|pre|td|ul|ol)>)!', '$1', $chunk);
+      $chunk = preg_replace('!<br />(\s*</?(?:p|li|div|dl|dd|dt|th|pre|td|ul|ol)>)!', '$1', $chunk);
       $chunk = preg_replace('/&([^#])(?![A-Za-z0-9]{1,8};)/', '&amp;$1', $chunk);
     }
     $output .= $chunk;

The problem with your patch is that you are making out-of-scope coding standards changes in your backport, in addition to changing just the two lines that were modified in #900956: Update autop filter to include missing block level tags. Plus, you coding standards changes are not proper (no upper case, no line terminator on inline comments).

As a minimum, this patch needs a re-roll to remove those out-of-scope changes. That will make the patch much easier to review properly.