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/regalloc/context.rs

@@ -58,8 +58,11 @@ impl Context {
         isa: &TargetIsa,
         func: &mut Function,
         cfg: &ControlFlowGraph,
-        domtree: &DominatorTree,
+        domtree: &mut DominatorTree,
     ) -> CtonResult {
+        // Ensure that a valid domtree exists.
+        domtree.ensure(func, cfg);
+
         // `Liveness` and `Coloring` are self-clearing.
         self.virtregs.clear();