Load: Only load PasswordHash class when needed.

This reverts [38371] which loaded `class-phpass.php` early in `wp-settings.php` and in turn caused backward compatibility problems.

Props DavidAnderson, ketuchetan.
Fixes #39445.


git-svn-id: https://develop.svn.wordpress.org/trunk@40387 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-login.php

@@ -438,6 +438,7 @@ case 'postpass' :
 		exit();
 	}
 
+	require_once ABSPATH . WPINC . '/class-phpass.php';
 	$hasher = new PasswordHash( 8, true );
 
 	/**