Fix LCA calculation in dom tree

hercules_ir/src/dom.rs

@@ -82,6 +82,9 @@ impl DomTree {
      * ancestor of the two nodes that is as far down the tree as possible.
      */
     pub fn least_common_ancestor(&self, mut a: NodeID, mut b: NodeID) -> NodeID {
+        if a == self.root || b == self.root {
+            return self.root;
+        }
         while self.idom[&a].0 > self.idom[&b].0 {
             a = self.idom[&a].1;
         }