dart2js cps: Redundant join elimination.

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

Index: pkg/compiler/lib/src/cps_ir/optimizers.dart
diff --git a/pkg/compiler/lib/src/cps_ir/optimizers.dart b/pkg/compiler/lib/src/cps_ir/optimizers.dart
index c80dfb0499f952cee3c110f286d231a4dbb2200e..3b48349f6e0e45d7633f8bf4e6ac7853043a22e6 100644
--- a/pkg/compiler/lib/src/cps_ir/optimizers.dart
+++ b/pkg/compiler/lib/src/cps_ir/optimizers.dart
@@ -5,9 +5,11 @@
 library dart2js.cps_ir.optimizers;
 
 import 'cps_ir_nodes.dart';
+import '../constants/values.dart';
 
 export 'type_propagation.dart' show TypePropagator;
 export 'redundant_phi.dart' show RedundantPhiEliminator;
+export 'redundant_join.dart' show RedundantJoinEliminator;
 export 'shrinking_reductions.dart' show ShrinkingReducer, ParentVisitor;
 
 /// An optimization pass over the CPS IR.
@@ -17,3 +19,15 @@ abstract class Pass {
 
   String get passName;
 }
+
+// Shared code between optimizations
+
+/// Returns true if [value] is false, null, 0, -0, NaN, or the empty string.
+bool isFalsyConstant(ConstantValue value) {
+  return value.isFalse ||
+      value.isNull  ||
+      value.isZero ||
+      value.isMinusZero ||
+      value.isNaN ||
+      value is StringConstantValue && value.primitiveValue.isEmpty;
+}