Add support for XMM8..XMM15

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index cebc86547d2ac1552c193bf276e163f74dfba6fd..c4a8ccf26fa64afc16a0e2b265a195e87f0e0cbc 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -676,6 +676,7 @@ class Assembler : public ValueObject {
   inline void EmitRegisterOperand(int rm, int reg);
   inline void EmitOperandREX(int rm, const Operand& operand, uint8_t rex);
   inline void EmitXmmRegisterOperand(int rm, XmmRegister reg);
+  inline void EmitXmmRegisterREX(XmmRegister reg, uint8_t rex);
   inline void EmitFixup(AssemblerFixup* fixup);
   inline void EmitOperandSizeOverride();
 
@@ -718,6 +719,13 @@ inline void Assembler::EmitRegisterREX(Register reg, uint8_t rex) {
 }
 
 
+inline void Assembler::EmitXmmRegisterREX(XmmRegister reg, uint8_t rex) {
+  ASSERT(reg != kNoXmmRegister);
+  rex |= (reg > 7 ? REX_R : REX_NONE);
+  if (rex != REX_NONE) EmitUint8(REX_PREFIX | rex);
+}
+
+
 inline void Assembler::EmitOperandREX(int rm,
                                       const Operand& operand,
                                       uint8_t rex) {