LibWeb: Replace is<T>() with as_if<T>() where possible

Author: https://github.com/tcl3
Commit: cb1a1a5cb5
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5985
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/HTML/Parser/HTMLParser.cpp

@@ -766,8 +766,8 @@ HTMLParser::AdjustedInsertionLocation HTMLParser::find_appropriate_place_for_ins
 
     // 3. If the adjusted insertion location is inside a template element,
     //    let it instead be inside the template element's template contents, after its last child (if any).
-    if (is<HTMLTemplateElement>(*adjusted_insertion_location.parent))
-        adjusted_insertion_location = { static_cast<HTMLTemplateElement const&>(*adjusted_insertion_location.parent).content().ptr(), nullptr };
+    if (auto* template_element = as_if<HTMLTemplateElement>(*adjusted_insertion_location.parent))
+        adjusted_insertion_location = { template_element->content().ptr(), nullptr };
 
     // 4. Return the adjusted insertion location.
     return adjusted_insertion_location;