dart2js cps: Add helpers for common IR manipulation.

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

Index: pkg/compiler/lib/src/cps_ir/scalar_replacement.dart
diff --git a/pkg/compiler/lib/src/cps_ir/scalar_replacement.dart b/pkg/compiler/lib/src/cps_ir/scalar_replacement.dart
index 18bb14d827a65c9f4000ec27200744011117b775..b9b6b09c378c616acfcca8d7b99027923a292bfc 100644
--- a/pkg/compiler/lib/src/cps_ir/scalar_replacement.dart
+++ b/pkg/compiler/lib/src/cps_ir/scalar_replacement.dart
@@ -141,11 +141,11 @@ class ScalarReplacementVisitor extends RecursiveVisitor {
         initialValue = new Constant(new NullConstantValue());
         LetPrim let = new LetPrim(initialValue);
         let.primitive.parent = let;
-        insertionPoint = insertAtBody(insertionPoint, let);
+        insertionPoint = let..insertBelow(insertionPoint);
       }
       LetMutable let = new LetMutable(variable, initialValue);
       let.value.parent = let;
-      insertionPoint = insertAtBody(insertionPoint, let);
+      insertionPoint = let..insertBelow(insertionPoint);
     }
 
     // Replace references with MutableVariable operations or references to the
@@ -188,15 +188,6 @@ class ScalarReplacementVisitor extends RecursiveVisitor {
     deleteLetPrimOf(allocation);
   }
 
-  InteriorNode insertAtBody(
-      InteriorNode insertionPoint, InteriorExpression let) {
-    let.parent = insertionPoint;
-    let.body = insertionPoint.body;
-    let.body.parent = let;
-    insertionPoint.body = let;
-    return let;
-  }
-
   /// Replaces [old] with [primitive] in [old]'s parent [LetPrim].
   void replacePrimitive(Primitive old, Primitive primitive) {
     LetPrim letPrim = old.parent;
@@ -206,11 +197,7 @@ class ScalarReplacementVisitor extends RecursiveVisitor {
   void deleteLetPrimOf(Primitive primitive) {
     assert(primitive.hasNoUses);
     LetPrim letPrim = primitive.parent;
-    Node child = letPrim.body;
-    InteriorNode parent = letPrim.parent;
-    child.parent = parent;
-    parent.body  = child;
-
+    letPrim.remove();
     deletePrimitive(primitive);
   }