minor symfony#26423 [DomCrawler] minor performance improvement (dunglas)

This PR was squashed before being merged into the 4.1-dev branch (closes symfony#26423).

Discussion
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[DomCrawler] minor performance improvement

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? |no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | n/a

Commits
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eead301 [DomCrawler] minor performance improvement

Author: nicolas-grekas

src/Symfony/Component/DomCrawler/Crawler.php

@@ -1080,8 +1080,9 @@ protected function sibling($node, $siblingDir = 'nextSibling')
     {
         $nodes = array();
 
+        $currentNode = $this->getNode(0);
         do {
-            if ($node !== $this->getNode(0) && 1 === $node->nodeType) {
+            if ($node !== $currentNode && XML_ELEMENT_NODE === $node->nodeType) {
                 $nodes[] = $node;
             }
         } while ($node = $node->$siblingDir);