Add guard against self assignment.

src/hydrogen.cc

 // Copyright 2012 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 1346 matching lines @@
   memset(data_, 0, kNumberOfTrackedSideEffects * kPointerSize);
 }
 
 
 HSideEffectMap::HSideEffectMap(HSideEffectMap* other) : count_(other->count_) {
   *this = *other;  // Calls operator=.
 }
 
 
 HSideEffectMap& HSideEffectMap::operator= (const HSideEffectMap& other) {
-  memcpy(data_, other.data_, kNumberOfTrackedSideEffects * kPointerSize);
+  if (this != &other) {
+    memcpy(data_, other.data_, kNumberOfTrackedSideEffects * kPointerSize);
+  }
   return *this;
 }
 
 void HSideEffectMap::Kill(GVNFlagSet flags) {
   for (int i = 0; i < kNumberOfTrackedSideEffects; i++) {
     GVNFlag changes_flag = HValue::ChangesFlagFromInt(i);
     if (flags.Contains(changes_flag)) {
       if (data_[i] != NULL) count_--;
       data_[i] = NULL;
     }
@@ skipping 7749 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal