VM: x64: Use correct register in write barrier to avoid an extra loop iteration

runtime/vm/stub_code_x64.cc

 // Copyright (c) 2013, 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/compiler.h"
 #include "vm/dart_entry.h"
@@ skipping 988 matching lines @@
   __ pushq(RAX);
   __ pushq(RCX);
 
   Label add_to_buffer;
   // Check whether this object has already been remembered. Skip adding to the
   // store buffer if the object is in the store buffer already.
   // Spilled: RAX, RCX
   // RDX: Address being stored
   Label reload;
   __ Bind(&reload);
-  __ movq(RCX, FieldAddress(RDX, Object::tags_offset()));
-  __ testq(RCX, Immediate(1 << RawObject::kRememberedBit));
+  __ movq(RAX, FieldAddress(RDX, Object::tags_offset()));
+  __ testq(RAX, Immediate(1 << RawObject::kRememberedBit));
   __ j(EQUAL, &add_to_buffer, Assembler::kNearJump);
   __ popq(RCX);
   __ popq(RAX);
   __ ret();
 
   // Update the tags that this object has been remembered.
   // RDX: Address being stored
   // RAX: Current tag value
   __ Bind(&add_to_buffer);
   __ movq(RCX, RAX);
@@ skipping 1255 matching lines @@
 }
 
 
 void StubCode::GenerateFrameAwaitingMaterializationStub(Assembler* assembler) {
   __ int3();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64