cps-ir: Merge variables based on set-based liveness and graph coloring.

tests/language/catch_liveness_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+@AssumeDynamic @NoInline
+foo() => 1;
+
+@AssumeDynamic @NoInline
+throwException() => throw 'x';
+
+main() {
+        var x = 10;
+        var e2 = null;
+        try {
+                var t = foo();
+                throwException();
+                print(t);
+                x = 3;
+        } catch(e) {
+                Expect.equals(10, x);
+                e2 = e;
+        }
+        Expect.equals(10, x);
+        Expect.equals('x', e2);
+}