Cleanup

hercules_ir/src/gcm.rs

-extern crate bitvec;
-
 use std::collections::{HashMap, HashSet, VecDeque};
 use std::iter::{zip, FromIterator};
 
@@ -183,6 +181,8 @@ pub fn gcm(
         // node.
         let schedule_early = schedule_early[id.idx()].unwrap();
         let mut chain = dom
+            // If the node has no users, then it doesn't really matter where we
+            // place it - just place it at the early placement.
             .chain(lca.unwrap_or(schedule_early), schedule_early)
             // Filter the dominator chain by control nodes in the same partition
             // as this data node, if the data node is in a partition already.