Add views_dependent_filters support for combined filters

views_dependent_filters allows exposed filters to be hidden and to prevent the filter being included in the database query it clears the filter value.

For quite a few filter types this works fine, but for combined filters the filter is still included in the query.

For my use-case I'm using op_regex() which can be made to work as follows, similar code could be added to the other op_*() functions.

handlers/views_handler_filter_combine.inc
   function op_regex($field) {
+    if (empty($this->value)) {
+      return;
+    }
   ...
   }