Overhaul tree IR visitor and rename IR classes.

pkg/compiler/lib/src/cps_ir/redundant_phi.dart

Index: pkg/compiler/lib/src/cps_ir/redundant_phi.dart
diff --git a/pkg/compiler/lib/src/cps_ir/redundant_phi.dart b/pkg/compiler/lib/src/cps_ir/redundant_phi.dart
index b61e7854b7a4a7d8c5e7c1635bb19073789a4744..0fe59ba9ad9e313e0ff29adfee99f5aff09899de 100644
--- a/pkg/compiler/lib/src/cps_ir/redundant_phi.dart
+++ b/pkg/compiler/lib/src/cps_ir/redundant_phi.dart
@@ -12,13 +12,14 @@ part of dart2js.cps_ir.optimizers;
 /// (except for feedback). Redundant parameters are removed from the
 /// continuation signature, all invocations, and replaced within the
 /// continuation body.
-class RedundantPhiEliminator extends RecursiveVisitor with PassMixin {
+class RedundantPhiEliminator extends RecursiveVisitor implements Pass {
   String get passName => 'Redundant phi elimination';
 
   final Set<Continuation> workSet = new Set<Continuation>();
 
   @override
-  void rewriteExecutableDefinition(final ExecutableDefinition root) {
+  void rewrite(RootNode root) {
+    if (root.isEmpty) return;
 
     // Set all parent pointers.
     new ParentVisitor().visit(root);