VM: Fix bug in x64 allocation tracing code.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index 7477651e67e5ae985df53a16da4e542fcb9176f5..1ed94881d5b3e3eb371d2da0f7955747ca5606c3 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -3492,6 +3492,7 @@ void Assembler::MaybeTraceAllocation(intptr_t cid,
     // temp_reg gets address of class table pointer.
     ClassTable* class_table = Isolate::Current()->class_table();
     movq(temp_reg, Immediate(class_table->ClassStatsTableAddress()));
+    movq(temp_reg, Address(temp_reg, 0));
     state_address = Address(temp_reg, class_offset + state_offset);
   }
   testb(state_address, Immediate(ClassHeapStats::TraceAllocationMask()));