Implement quadword MOV on x64 assembler, emitting REX prefix.

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 797 matching lines @@
   }
 
   // code emission
   void GrowBuffer();
   inline void emit(uint32_t x);
   inline void emit(Handle<Object> handle);
   inline void emit(uint32_t x, RelocInfo::Mode rmode);
   inline void emit(const Immediate& x);
   inline void emit_w(const Immediate& x);
 
+  // Emits a REX prefix that encodes a 64-bit operand size and
+  // the top bit of both register codes.
+  inline void emit_rex_64(Register reg, Register rm_reg);
+
+  // Emits a REX prefix that encodes a 64-bit operand size and
+  // the top bit of the destination, index, and base register codes.
+  inline void emit_rex_64(Register reg, const Operand& op);
+
   // Emit the code-object-relative offset of the label's position
   inline void emit_code_relative_offset(Label* label);
 
   // instruction generation
   void emit_arith_b(int op1, int op2, Register dst, int imm8);
 
   // Emit a basic arithmetic instruction (i.e. first byte of the family is 0x81)
   // with a given destination expression and an immediate operand.  It attempts
   // to use the shortest encoding possible.
   // sel specifies the /n in the modrm byte (see the Intel PRM).
@@ skipping 66 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_