LoadCodeUnitsInstr can be passed both tagged and untagged string input.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index 02812cb6bf686de24a81aef70eac0bb8d6d62682..9e83d1fd24844b682dae9b79b891a02719b313e6 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -3710,6 +3710,15 @@ class LoadCodeUnitsInstr : public TemplateDefinition<2, NoThrow> {
   DECLARE_INSTRUCTION(LoadCodeUnits)
   virtual CompileType ComputeType() const;
 
+  virtual Representation RequiredInputRepresentation(intptr_t idx) const {
+    if (idx == 0) {
+      // The string may be tagged or untagged (for external strings).
+      return kNoRepresentation;
+    }
+    ASSERT(idx == 1);
+    return kTagged;
+  }
+
   bool IsExternal() const {
     return array()->definition()->representation() == kUntagged;
   }