Use register allocator for context on x64.

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // 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.
 //
@@ skipping 73 matching lines @@
 // to a few constants). If this is a problem, we could change the code
 // such that we use an enum in optimized mode, and the struct in debug
 // mode. This way we get the compile-time error checking in debug mode
 // and best performance in optimized code.
 //
 
 struct Register {
   // The non-allocatable registers are:
   //  rsp - stack pointer
   //  rbp - frame pointer
-  //  rsi - context register
   //  r10 - fixed scratch register
   //  r12 - smi constant register
   //  r13 - root register
-  static const int kMaxNumAllocatableRegisters = 10;
+  static const int kMaxNumAllocatableRegisters = 11;
   static int NumAllocatableRegisters() {
     return kMaxNumAllocatableRegisters;
   }
   static const int kNumRegisters = 16;
 
   static int ToAllocationIndex(Register reg) {
     return kAllocationIndexByRegisterCode[reg.code()];
   }
 
   static Register FromAllocationIndex(int index) {
     ASSERT(index >= 0 && index < kMaxNumAllocatableRegisters);
     Register result = { kRegisterCodeByAllocationIndex[index] };
     return result;
   }
 
   static const char* AllocationIndexToString(int index) {
     ASSERT(index >= 0 && index < kMaxNumAllocatableRegisters);
     const char* const names[] = {
       "rax",
       "rbx",
       "rdx",
       "rcx",
+      "rsi",
       "rdi",
       "r8",
       "r9",
       "r11",
       "r14",
       "r15"
     };
     return names[index];
   }
 
@@ skipping 1573 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_