Move vm_tags from isolate to thread, since we may have multiple threads in same isolate (GC, backgr…

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index 89b294497c32a69d2eb553b63f3f03abed3e1187..0a6b694a5e6e99e85d6bf85c8240d00ea0690492 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -1032,6 +1032,10 @@ class Assembler : public ValueObject {
                                            Register array,
                                            Register index);
 
+  static Address VMTagAddress() {
+    return Address(THR, Thread::vm_tag_offset());
+  }
+
   // On some other platforms, we draw a distinction between safe and unsafe
   // smis.
   static bool IsSafe(const Object& object) { return true; }