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

runtime/vm/assembler_mips.h

Index: runtime/vm/assembler_mips.h
diff --git a/runtime/vm/assembler_mips.h b/runtime/vm/assembler_mips.h
index 714e2d8c923fef2d479cc739200c7c7603aa432d..531611032dff834a6af7e8a3f0f5b0d143b274d5 100644
--- a/runtime/vm/assembler_mips.h
+++ b/runtime/vm/assembler_mips.h
@@ -1593,6 +1593,10 @@ 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; }