Add missing assert

Change-Id: I951254cc8917cf6df3522327b40f628661ecff32
diff --git a/src/main/java/com/android/tools/r8/ir/code/IRCode.java b/src/main/java/com/android/tools/r8/ir/code/IRCode.java
index 7e2e851..9c0a66b 100644
--- a/src/main/java/com/android/tools/r8/ir/code/IRCode.java
+++ b/src/main/java/com/android/tools/r8/ir/code/IRCode.java
@@ -195,7 +195,7 @@
         }
         for (Value value : phi.getDebugValues()) {
           values.add(value);
-          value.debugPhiUsers().contains(phi);
+          assert value.debugPhiUsers().contains(phi);
         }
       }
       for (Instruction instruction : block.getInstructions()) {