Split GuardField into GuardFieldType and GuardFieldLength instructions.

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 abd993375d13cc52958c7510f3a56ee4b172b951..dd7f08ca7649a4685e59567864d598fdf547bc40 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -685,6 +685,11 @@ class RawField : public RawObject {
   intptr_t guarded_cid_;
   intptr_t is_nullable_;  // kNullCid if field can contain null value and
                           // any other value otherwise.
+  // Offset to the guarded length field inside an instance of class matching
+  // guarded_cid_. Stored corrected by -kHeapObjectTag to simplify code
+  // generated on platforms with weak addressing modes (ARM, MIPS).
+  int8_t guarded_list_length_in_object_offset_;
+
   uint8_t kind_bits_;  // static, final, const, has initializer.
 };