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

runtime/vm/stub_code_x64.cc

Index: runtime/vm/stub_code_x64.cc
diff --git a/runtime/vm/stub_code_x64.cc b/runtime/vm/stub_code_x64.cc
index 0341f268f80c2de6cae68d9a28c65721deed2864..b3b18da03bced6a40ab816a2f32e0ad70fb3dc79 100644
--- a/runtime/vm/stub_code_x64.cc
+++ b/runtime/vm/stub_code_x64.cc
@@ -1006,8 +1006,8 @@ void StubCode::GenerateUpdateStoreBufferStub(Assembler* assembler) {
   // 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);