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/HTMLImageElement.cpp

@@ -1087,8 +1087,8 @@ static void update_the_source_set(DOM::Element& element)
     VERIFY(is<HTMLImageElement>(element) || is<HTMLLinkElement>(element));
 
     // 1. Set el's source set to an empty source set.
-    if (is<HTMLImageElement>(element))
-        static_cast<HTMLImageElement&>(element).set_source_set(SourceSet {});
+    if (auto* image_element = as_if<HTMLImageElement>(element))
+        image_element->set_source_set(SourceSet {});
     else if (is<HTMLLinkElement>(element))
         TODO();