Coding Standards: Use pre-increment/decrement for stand-alone statements.

Note: This is enforced by WPCS 3.0.0:

1. There should be no space between an increment/decrement operator and the variable it applies to.
2. Pre-increment/decrement should be favoured over post-increment/decrement for stand-alone statements. “Pre” will in/decrement and then return, “post” will return and then in/decrement. Using the “pre” version is slightly more performant and can prevent future bugs when code gets moved around.

References:
* [https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#increment-decrement-operators WordPress PHP Coding Standards: Increment/decrement operators]
* [https://github.com/WordPress/WordPress-Coding-Standards/pull/2130 WPCS: PR #2130 Core: add sniffs to check formatting of increment/decrement operators]

Props jrf.
See #59161, #58831.
Built from https://develop.svn.wordpress.org/trunk@56549


git-svn-id: http://core.svn.wordpress.org/trunk@56061 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/media.php

@@ -638,7 +638,7 @@ function wp_iframe( $content_func, ...$args ) {
  */
 function media_buttons( $editor_id = 'content' ) {
 	static $instance = 0;
-	$instance++;
+	++$instance;
 
 	$post = get_post();