Use a root array register for constant loads and stack guards on x64....

src/x64/cfg-x64.cc

 // Copyright 2009 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 53 matching lines @@
   is_marked_ = true;
   Label deferred_enter, deferred_exit;
   {
     Comment cmnt(masm, "[ EntryNode");
     __ push(rbp);
     __ movq(rbp, rsp);
     __ push(rsi);
     __ push(rdi);
     int count = CfgGlobals::current()->fun()->scope()->num_stack_slots();
     if (count > 0) {
-      __ movq(kScratchRegister, Factory::undefined_value(),
-              RelocInfo::EMBEDDED_OBJECT);
+      __ LoadRoot(kScratchRegister, Heap::kUndefinedValueRootIndex);
       for (int i = 0; i < count; i++) {
         __ push(kScratchRegister);
       }
     }
     if (FLAG_trace) {
       __ CallRuntime(Runtime::kTraceEnter, 0);
     }
     if (FLAG_check_stack) {
       ExternalReference stack_limit =
           ExternalReference::address_of_stack_guard_limit();
@@ skipping 228 matching lines @@
       break;
     case NOT_ALLOCATED:
       UNREACHABLE();
   }
 }
 
 
 #undef __
 
 } }  // namespace v8::internal