The uninitialized value is immortal immovable.

src/hydrogen-instructions.h

 // 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 3392 matching lines @@
     if (has_external_reference_value_) {
       return false;
     }
 
     ASSERT(!object_.handle().is_null());
     Heap* heap = isolate()->heap();
     ASSERT(!object_.IsKnownGlobal(heap->minus_zero_value()));
     ASSERT(!object_.IsKnownGlobal(heap->nan_value()));
     return
         object_.IsKnownGlobal(heap->undefined_value()) ||
+        object_.IsKnownGlobal(heap->uninitialized_value()) ||
         object_.IsKnownGlobal(heap->null_value()) ||
         object_.IsKnownGlobal(heap->true_value()) ||
         object_.IsKnownGlobal(heap->false_value()) ||
         object_.IsKnownGlobal(heap->the_hole_value()) ||
         object_.IsKnownGlobal(heap->empty_string()) ||
         object_.IsKnownGlobal(heap->empty_fixed_array());
   }
 
   bool IsCell() const {
     return is_cell_;
@@ skipping 3983 matching lines @@
   virtual bool IsDeletable() const V8_OVERRIDE { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_