[x64] Fix code generation of testw and add tests

src/x64/assembler-x64.cc

 // 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.
 
 #include "src/x64/assembler-x64.h"
 
 #include <cstring>
 
 #if V8_TARGET_ARCH_X64
 
@@ skipping 2085 matching lines @@
   } else {
     emit_optional_rex_32(reg, op);
   }
   emit(0x84);
   emit_operand(reg, op);
 }
 
 void Assembler::testw(Register dst, Register src) {
   EnsureSpace ensure_space(this);
   emit(0x66);
-  if (src.low_bits() == 4) {
-    emit_rex_32(src, dst);
+  if (!dst.is_byte_register() || !src.is_byte_register()) {
+    // Register is not one of al, bl, cl, dl.  Its encoding needs REX.
+    emit_rex_32(dst, src);
   }
   emit(0x85);
-  emit_modrm(src, dst);
+  emit_modrm(dst, src);
 }
 
 void Assembler::testw(Register reg, Immediate mask) {
   DCHECK(is_int16(mask.value_) || is_uint16(mask.value_));
   EnsureSpace ensure_space(this);
   emit(0x66);
   if (reg.is(rax)) {
     emit(0xA9);
     emitw(mask.value_);
   } else {
-    if (reg.low_bits() == 4) {
+    if (!reg.is_byte_register()) {
       emit_rex_32(reg);
     }
     emit(0xF7);
     emit_modrm(0x0, reg);
     emitw(mask.value_);
   }
 }
 
 void Assembler::testw(const Operand& op, Immediate mask) {
   DCHECK(is_int16(mask.value_) || is_uint16(mask.value_));
@@ skipping 2624 matching lines @@
 
 bool RelocInfo::IsInConstantPool() {
   return false;
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64