Recompute the dominator tree on demand.

The legalizer can invalidate the dominator tree, but we don't actually
need a dominator tree during legalization, so defer the construction of
the domtree.

- Add an "invalid" state to the dominator tree along with clear() and
  is_valid() methods to test it.
- Invalidate the dominator tree as part of legalization.
- Ensure that a valid dominator tree exists before the passes that need
  it.

Together these features add up to a manual invalidation mechanism for
the dominator tree.

lib/cretonne/src/verifier/mod.rs

@@ -134,7 +134,9 @@ pub fn verify_context(func: &Function,
                       -> Result {
     let verifier = Verifier::new(func, isa);
     verifier.cfg_integrity(cfg)?;
-    verifier.domtree_integrity(domtree)?;
+    if domtree.is_valid() {
+        verifier.domtree_integrity(domtree)?;
+    }
     verifier.run()
 }