[x64] Make xmm0 allocatable and use xmm15 as scratch register instead.

src/x64/macro-assembler-x64.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_X64_MACRO_ASSEMBLER_X64_H_
 #define V8_X64_MACRO_ASSEMBLER_X64_H_
 
 #include "src/assembler.h"
 #include "src/bailout-reason.h"
 #include "src/base/flags.h"
@@ skipping 16 matching lines @@
 const Register kInterpreterBytecodeArrayRegister = {Register::kCode_r14};
 const Register kInterpreterDispatchTableRegister = {Register::kCode_r15};
 const Register kJavaScriptCallArgCountRegister = {Register::kCode_rax};
 const Register kJavaScriptCallNewTargetRegister = {Register::kCode_rdx};
 const Register kRuntimeCallFunctionRegister = {Register::kCode_rbx};
 const Register kRuntimeCallArgCountRegister = {Register::kCode_rax};
 
 // Default scratch register used by MacroAssembler (and other code that needs
 // a spare register). The register isn't callee save, and not used by the
 // function calling convention.
-const Register kScratchRegister = { 10 };      // r10.
-const Register kRootRegister = { 13 };         // r13 (callee save).
+const Register kScratchRegister = {10};      // r10.
+const XMMRegister kScratchDoubleReg = {15};  // xmm15.
+const Register kRootRegister = {13};         // r13 (callee save).
 // Actual value of root register is offset from the root array's start
 // to take advantage of negitive 8-bit displacement values.
 const int kRootRegisterBias = 128;
 
 // Convenience for platform-independent signatures.
 typedef Operand MemOperand;
 
 enum RememberedSetAction { EMIT_REMEMBERED_SET, OMIT_REMEMBERED_SET };
 enum SmiCheck { INLINE_SMI_CHECK, OMIT_SMI_CHECK };
 enum PointersToHereCheck {
@@ skipping 1724 matching lines @@
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_