Revert of [x64] Zero/sign-extend loads to 64-bit registers.

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 690 matching lines @@
   void movp(Register dst, void* ptr, RelocInfo::Mode rmode);
 
   // Loads a 64-bit immediate into a register.
   void movq(Register dst, int64_t value,
             RelocInfo::Mode rmode = RelocInfo::NONE64);
   void movq(Register dst, uint64_t value,
             RelocInfo::Mode rmode = RelocInfo::NONE64);
 
   void movsxbl(Register dst, Register src);
   void movsxbl(Register dst, const Operand& src);
-  void movsxbq(Register dst, Register src);
   void movsxbq(Register dst, const Operand& src);
   void movsxwl(Register dst, Register src);
   void movsxwl(Register dst, const Operand& src);
-  void movsxwq(Register dst, Register src);
   void movsxwq(Register dst, const Operand& src);
   void movsxlq(Register dst, Register src);
   void movsxlq(Register dst, const Operand& src);
 
   // Repeated moves.
 
   void repmovsb();
   void repmovsw();
   void repmovsp() { emit_repmovs(kPointerSize); }
   void repmovsl() { emit_repmovs(kInt32Size); }
@@ skipping 1620 matching lines @@
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_X64_ASSEMBLER_X64_H_