* Move the old sidebars_widgets array upgrade code to wp-admin/includes/upgrade.php

* Avoid infinite loop with wp_get_sidebars_widgets()
* Remove two unneeded wp_get_sidebars_widgets() calls
* Remove unused $wp_registered_sidebars variable from wp_get_sidebars_widgets().
* Combine  a couple of !empty( $sidebars_widgets ) checks in retrieve_widgets()
Props SergeyBiryukov, lancewillett. fixes #17979 #18598


git-svn-id: https://core.svn.wordpress.org/trunk@18821 1a063a9b-81f0-0310-95a4-ce76da25c4cd