Fix bug in the x64 assembler's movw instruction.

runtime/vm/assembler_x64.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/heap.h"
 #include "vm/memory_region.h"
@@ skipping 277 matching lines @@
 void Assembler::movsxw(Register dst, const Address& src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitOperandREX(dst, src, REX_W);
   EmitUint8(0x0F);
   EmitUint8(0xBF);
   EmitOperand(dst & 7, src);
 }
 
 
 void Assembler::movw(Register dst, const Address& src) {
-  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
-  EmitOperandREX(dst, src, REX_NONE);
-  EmitOperandSizeOverride();
-  EmitUint8(0x8B);
-  EmitOperand(dst & 7, src);
+  FATAL("Use movzxw or movsxw instead.");
 }
 
 
 void Assembler::movw(const Address& dst, Register src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitOperandSizeOverride();
   EmitOperandREX(src, dst, REX_NONE);
-  EmitOperandSizeOverride();
   EmitUint8(0x89);
   EmitOperand(src & 7, dst);
 }
 
 
 void Assembler::movq(Register dst, const Immediate& imm) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   if (imm.is_int32()) {
     Operand operand(dst);
     EmitOperandREX(0, operand, REX_W);
@@ skipping 1870 matching lines @@
 
 const char* Assembler::XmmRegisterName(XmmRegister reg) {
   ASSERT((0 <= reg) && (reg < kNumberOfXmmRegisters));
   return xmm_reg_names[reg];
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64