Improve domtree as per @Amanieu's feedback.

fuzz/fuzz_targets/domtree.rs

@@ -85,16 +85,13 @@ fn check_idom_violations(idom: &[Block], path: &Path) {
         let mut parent = idom[block.index()];
         let mut domset = HashSet::new();
         domset.insert(*block);
-        loop {
+        while parent.is_valid() {
-            assert!(parent.is_valid());
             assert!(visited.contains(&parent));
             domset.insert(parent);
             let next = idom[parent.index()];
-            if next == parent {
-                break;
-            }
             parent = next;
         }
+
         // Check that `dominates()` returns true for every block in domset,
         // and false for every other block.
         for domblock in 0..idom.len() {

src/domtree.rs

@@ -98,6 +98,10 @@ pub fn calculate<'a, PredFn: Fn(Block) -> &'a [Block]>(
         }
     }
 
+    // Now set the start node's dominator-tree parent to "invalid";
+    // this allows the loop in `dominates` to terminate.
+    idom[start.index()] = Block::invalid();
+
     idom
 }
 
@@ -109,10 +113,6 @@ pub fn dominates(idom: &[Block], a: Block, mut b: Block) -> bool {
         if b.is_invalid() {
             return false;
         }
-        let parent = idom[b.index()];
-        if b == parent {
-            return false;
-        }
         b = idom[b.index()];
     }
 }