Add instruction tags to saved ICData (debug mode only).

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 323fed40e58b2e0aa402bd6977222a52ee3306d0..e66204f1df17585aa79033d14de3bde28492bbf1 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1478,6 +1478,10 @@ class RawICData : public RawObject {
   int32_t deopt_id_;     // Deoptimization id corresponding to this IC.
   uint32_t state_bits_;  // Number of arguments tested in IC, deopt reasons,
                          // range feedback.
+#if defined(TAG_IC_DATA)
+  intptr_t tag_;  // Debugging, verifying that the icdata is assigned to the
+                  // same instruction again. Store -1 or Instruction::Tag.
+#endif
 };