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/database/DB.php b/system/database/DB.php
index d910474..83d9733 100644
--- a/system/database/DB.php
+++ b/system/database/DB.php
@@ -149,7 +149,7 @@
 	if ( ! isset($query_builder) OR $query_builder === TRUE)
 	{
 		require_once(BASEPATH.'database/DB_query_builder.php');
-		if ( ! class_exists('CI_DB'))
+		if ( ! class_exists('CI_DB', FALSE))
 		{
 			/**
 			 * CI_DB
@@ -162,7 +162,7 @@
 			class CI_DB extends CI_DB_query_builder { }
 		}
 	}
-	elseif ( ! class_exists('CI_DB'))
+	elseif ( ! class_exists('CI_DB', FALSE))
 	{
 		/**
 	 	 * @ignore
diff --git a/system/database/DB_driver.php b/system/database/DB_driver.php
index 35ac8e8..213e2c3 100644
--- a/system/database/DB_driver.php
+++ b/system/database/DB_driver.php
@@ -704,7 +704,7 @@
 	{
 		$driver = 'CI_DB_'.$this->dbdriver.'_result';
 
-		if ( ! class_exists($driver))
+		if ( ! class_exists($driver, FALSE))
 		{
 			include_once(BASEPATH.'database/DB_result.php');
 			include_once(BASEPATH.'database/drivers/'.$this->dbdriver.'/'.$this->dbdriver.'_result.php');
@@ -1560,7 +1560,7 @@
 	 */
 	protected function _cache_init()
 	{
-		if (class_exists('CI_DB_Cache'))
+		if (class_exists('CI_DB_Cache', FALSE))
 		{
 			if (is_object($this->CACHE))
 			{