Allow HPhis to have an invalid merge index.

src/hydrogen-dce.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 100 matching lines @@
         phi->ClearFlag(HValue::kIsLive);
       }
     }
   }
 
   // Process phis separately to avoid simultaneously mutating the phi list.
   while (!worklist.is_empty()) {
     HPhi* phi = worklist.RemoveLast();
     HBasicBlock* block = phi->block();
     phi->DeleteAndReplaceWith(NULL);
-    block->RecordDeletedPhi(phi->merged_index());
+    if (phi->HasMergedIndex()) {
+      block->RecordDeletedPhi(phi->merged_index());
+    }
   }
 }
 
 } }  // namespace v8::internal