Bug 1891296 - Fix an assertion in selector-matching. r=dshin

We can have combinator sequences like [>, <part>], and they are fine.

Add a test to make sure they're handled correctly.

Differential Revision: https://phabricator.services.mozilla.com/D208668

servo/components/selectors/matching.rs

@@ -362,12 +362,7 @@ where
 
     let iter = selector.iter_from(selector.len() - from_offset);
     debug_assert!(
-        iter.clone().next().is_some() ||
-            (from_offset != selector.len() &&
-                matches!(
-                    selector.combinator_at_parse_order(from_offset),
-                    Combinator::SlotAssignment | Combinator::PseudoElement
-                )),
+        iter.clone().next().is_some() || from_offset != selector.len(),
         "Got the math wrong: {:?} | {:?} | {} {}",
         selector,
         selector.iter_raw_match_order().as_slice(),