Micro-optimize Language::isMultiByte()

The xenon logs on performance.wikimedia.org show this function as being on-CPU
about 1% of the time, making it a candidate for optimization. The version in
this patch is about 30-50% faster in my benchmarks.

Change-Id: I26385ade7600fc11965d94468b57e41ec257de51

languages/Language.php

@@ -2838,7 +2838,7 @@ class Language {
 	 * @return bool
 	 */
 	function isMultibyte( $str ) {
-		return (bool)preg_match( '/[\x80-\xff]/', $str );
+		return strlen( $str ) !== mb_strlen( $str );
 	}
 
 	/**