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

src/x64/register-allocator-x64.h

Index: src/x64/register-allocator-x64.h
===================================================================
--- src/x64/register-allocator-x64.h	(revision 2780)
+++ src/x64/register-allocator-x64.h	(working copy)
@@ -33,9 +33,7 @@
 
 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;
 };