General: use a new function, wp_is_IE(), instead of the $is_IE global in a number of places.

See #37699.

Built from https://develop.svn.wordpress.org/trunk@38467


git-svn-id: http://core.svn.wordpress.org/trunk@38408 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/vars.php

@@ -139,3 +139,15 @@ function wp_is_mobile() {
 
 	return $is_mobile;
 }
+
+/**
+ * @since 4.7.0
+ *
+ * @global bool $is_IE Whether the current user agent is Internet Explorer
+ *
+ * @return bool Return the value of the $is_IE global.
+ */
+function wp_is_IE() {
+	global $is_IE;
+	return $is_IE;
+}