[x64] Fix code generation of testw and add tests

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index 5402a8ce87a3987981e1c8a33340238b95b517c3..7a7fc6c87bcae85226d288b86a25060ab907ac49 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -2103,11 +2103,12 @@ void Assembler::testb(const Operand& op, Register reg) {
 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) {
@@ -2118,7 +2119,7 @@ void Assembler::testw(Register reg, Immediate mask) {
     emit(0xA9);
     emitw(mask.value_);
   } else {
-    if (reg.low_bits() == 4) {
+    if (!reg.is_byte_register()) {
       emit_rex_32(reg);
     }
     emit(0xF7);