Move ensure_domtree out of Context and into DominatorTree.

This also moves the calls to it out of Context and into the passes that
actually need it, so that Context's functions don't have any logic of
their own.

lib/cretonne/src/dominator_tree.rs

@@ -237,6 +237,14 @@ impl DominatorTree {
         !self.nodes.is_empty()
     }
 
+    /// Conveneince function to call `compute` if `compute` hasn't been called
+    /// since the last `clear()`.
+    pub fn ensure(&mut self, func: &Function, cfg: &ControlFlowGraph) {
+        if !self.is_valid() {
+            self.compute(func, cfg)
+        }
+    }
+
     /// Reset all internal data structures and compute a post-order for `cfg`.
     ///
     /// This leaves `rpo_number == 1` for all reachable EBBs, 0 for unreachable ones.