dart2js cps: Make Identical a built-in and add Interceptor to Tree IR.

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

Index: pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
diff --git a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
index eed6448075057df99c451595eb18a4e113df3063..fb62a729fe33195f03afce7a434c0e94b079bd55 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -616,11 +616,6 @@ class ParentVisitor extends RecursiveVisitor {
     node.value.parent = node;
   }
 
-  processIdentical(Identical node) {
-    node.left.parent = node;
-    node.right.parent = node;
-  }
-
   processInterceptor(Interceptor node) {
     node.input.parent = node;
   }