In `wp-includes/revision.php` - remove dead code:

* In `wp_get_post_autosave()`, `break` is unreachable after `return`
* In `_wp_put_post_revision()`, `$post_id` is set then never used.

See #27882.

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In `wp_default_scripts()`, `$max_upload_size` and its entangled children `$max_up` and `$max_post` create quite the ternary operator... that is never used.

See #27882.

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In `print_admin_styles()`, `$zip` is never used after being set. `$compress_css` does not need to be imported.

See #27882.

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In `wp-includes/theme.php`, `break` is unreachable after `return`.

See #27882.

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In `WP_Widget::update_callback()`, `$sidebars_widgets` is unused.

See #27882.

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In `wpdb`, remove dead code:

* In `->tables()`, `break` is unreachable after `return`
* In `->query()`, `$return` is always set, so doesn't need an initial value of `0`
* In `->delete()`, `$bits` is unused

See #27882.

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In `wp-login.php`, `break` is unreachable after `exit`

See #27882.

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Bundled themes: use correct logic in IE conditional comments in header template. Props mikemanger, fixes #28151.
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