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

src/x64/register-allocator-x64.h

 // 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
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@@ skipping 15 matching lines @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #ifndef V8_X64_REGISTER_ALLOCATOR_X64_H_
 #define V8_X64_REGISTER_ALLOCATOR_X64_H_
 
 namespace v8 {
 namespace internal {
 
 class RegisterAllocatorConstants : public AllStatic {
  public:
-  // Register allocation is not yet implemented on x64, but C++
-  // forbids 0-length arrays so we use 1 as the number of registers.
-  static const int kNumRegisters = 12;
+  static const int kNumRegisters = 11;
   static const int kInvalidRegister = -1;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_REGISTER_ALLOCATOR_X64_H_