simplify stackless iterator is_cyclic (#2111)

src/machine/gc.rs

@@ -272,14 +272,9 @@ impl<'a, UMP: UnmarkPolicy> StacklessPreOrderHeapIter<'a, UMP> {
     #[inline]
     fn is_cyclic(&self, var_current: usize, var_next: usize) -> bool {
         if self.heap[var_next].is_var() {
-            var_current != var_next && self.current + 1 != var_current
-        } else if self.heap[var_next].is_ref() {
-            // the cell var_next in the second branch contains its original
-            // value whether var_next is marked or unmarked, meaning the
-            // is_compound check is well-founded in either case.
-            self.heap[var_next].get_forwarding_bit()
+            !self.heap[var_next].get_forwarding_bit() && var_current != var_next
         } else {
-            false
+            self.heap[var_next].is_ref()
         }
     }