dart2js CPS: During shrinking reductions, mark deleted branches as deleted.

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 42ae6cda760e3632c55cde3e93e220b1dac4f276..acb5b91637f0d846bebfd8cf728a91f9dc3a33fb 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -132,8 +132,6 @@ class ShrinkingReducer extends Pass {
       case _ReductionKind.BRANCH:
         _reduceBranch(task);
         break;
-      default:
-        assert(false);
     }
   }
 
@@ -638,6 +636,10 @@ class _RemovalVisitor extends TrampolineRecursiveVisitor {
     node.parent = null;
   }
 
+  void processBranch(Branch node) {
+    node.parent = null;
+  }
+
   void processReference(Reference reference) {
     reference.unlink();