AK: Use simdutf to validate UTF-16 strings as ASCII

Author: https://github.com/trflynn89
Commit: 086877a280
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6001
Reviewed-by: https://github.com/gmta ✅

AK/Utf16View.cpp

@@ -151,9 +151,7 @@ bool Utf16View::is_ascii() const
 {
     if (has_ascii_storage())
         return true;
-
+    return simdutf::validate_utf16_as_ascii(m_string.utf16, length_in_code_units());
-    // FIXME: Petition simdutf to implement an ASCII validator for UTF-16.
-    return all_of(utf16_span(), AK::is_ascii);
 }
 
 bool Utf16View::validate() const