Add a system property to disable horizontal class merging

Bug: b/323220264
Change-Id: I6cac82163e51cb725478cb1460e2a51a51cf8959
diff --git a/src/main/java/com/android/tools/r8/utils/InternalOptions.java b/src/main/java/com/android/tools/r8/utils/InternalOptions.java
index f284b2c..6ea5e06 100644
--- a/src/main/java/com/android/tools/r8/utils/InternalOptions.java
+++ b/src/main/java/com/android/tools/r8/utils/InternalOptions.java
@@ -1822,12 +1822,9 @@
 
   public class HorizontalClassMergerOptions {
 
-    // TODO(b/138781768): Set enable to true when this bug is resolved.
     private boolean enable =
-        !Version.isDevelopmentVersion()
-            || System.getProperty("com.android.tools.r8.disableHorizontalClassMerging") == null;
+        System.getProperty("com.android.tools.r8.disableHorizontalClassMerging") == null;
     private boolean enableInitial = true;
-    // TODO(b/205611444): Enable by default.
     private boolean enableClassInitializerDeadlockDetection = true;
     private boolean enableInterfaceMerging =
         System.getProperty("com.android.tools.r8.enableHorizontalInterfaceMerging") != null;