Implement shrinking reductions in CPS IR

sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_nodes.dart

Index: sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_nodes.dart
diff --git a/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_nodes.dart b/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_nodes.dart
index c9a10e574e8f77d9bed751f6f22a545b8e033968..603334faa2a8231b67906174502d026fc4a59e68 100644
--- a/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_nodes.dart
+++ b/sdk/lib/_internal/compiler/implementation/cps_ir/cps_ir_nodes.dart
@@ -18,6 +18,9 @@ abstract class Node {
   static int hashCount = 0;
   final int hashCode = hashCount = (hashCount + 1) & 0x3fffffff;
 
+  /// A pointer to the parent node. Is null until set by optimization passes.
+  Node parent;
+
   accept(Visitor visitor);
 }
 
@@ -83,6 +86,9 @@ class Reference {
   Reference previous = null;
   Reference next = null;
 
+  /// A pointer to the parent node. Is null until set by optimization passes.
+  Node parent;
+
   Reference(this.definition) {
     next = definition.firstRef;
     if (next != null) next.previous = this;