Detect phi cycles.

Having reference and non-reference phi equivalent, only happened
for the 0/null constant. To avoid such occurences, we must
detect phi cycles.

bug:25493693

Change-Id: Ie1a8460c3abacca96c299da107fa4407e17dd792
diff --git a/test/557-checker-ref-equivalent/src/Main.java b/test/557-checker-ref-equivalent/src/Main.java
new file mode 100644
index 0000000..a970af5
--- /dev/null
+++ b/test/557-checker-ref-equivalent/src/Main.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+public class Main {
+
+  /// CHECK-START: void Main.testRedundantPhiCycle(boolean) ssa_builder (after)
+  /// CHECK-NOT:  Phi
+  private void testRedundantPhiCycle(boolean cond) {
+    Object o = null;
+    while (true) {
+      if (cond) {
+        o = null;
+      }
+      System.out.println(o);
+    }
+  }
+
+  /// CHECK-START: void Main.testLoopPhisWithNullAndCrossUses(boolean) ssa_builder (after)
+  /// CHECK-NOT:  Phi
+  private void testLoopPhisWithNullAndCrossUses(boolean cond) {
+    Main a = null;
+    Main b = null;
+    while (a == null) {
+      if (cond) {
+        a = b;
+      } else {
+        b = a;
+      }
+    }
+  }
+
+  public static void main(String[] args) {
+  }
+}