Disable autoloader call from class_exists() occurences to improve performance

Note: The Driver libary tests seem to depend on that, so one occurence in CI_Loader is left until we resolve that.
diff --git a/system/core/Output.php b/system/core/Output.php
index a208414..25ecd49 100644
--- a/system/core/Output.php
+++ b/system/core/Output.php
@@ -395,7 +395,7 @@
 		global $BM, $CFG;
 
 		// Grab the super object if we can.
-		if (class_exists('CI_Controller'))
+		if (class_exists('CI_Controller', FALSE))
 		{
 			$CI =& get_instance();
 		}