Box values of LoadCodeUnits that fall outside of smi range on 32bit platforms.

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 b92de0a9de275dcd022df81ca1008c41eb82943f..903d31906b9436b0d5663be2445d60b2fad9b477 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -3701,7 +3701,8 @@ class LoadCodeUnitsInstr : public TemplateDefinition<2, NoThrow> {
                      intptr_t token_pos)
       : class_id_(class_id),
         token_pos_(token_pos),
-        element_count_(element_count) {
+        element_count_(element_count),
+        representation_(kTagged) {
     ASSERT(element_count == 1 || element_count == 2 || element_count == 4);
     ASSERT(class_id == kOneByteStringCid || class_id == kTwoByteStringCid);
     SetInputAt(0, str);
@@ -3729,7 +3730,8 @@ class LoadCodeUnitsInstr : public TemplateDefinition<2, NoThrow> {
 
   virtual bool CanDeoptimize() const { return false; }
 
-  virtual Representation representation() const;
+  virtual Representation representation() const { return representation_; }
+  void set_representation(Representation repr) { representation_ = repr; }
   virtual void InferRange(RangeAnalysis* analysis, Range* range);
 
   virtual EffectSet Effects() const { return EffectSet::None(); }
@@ -3738,6 +3740,7 @@ class LoadCodeUnitsInstr : public TemplateDefinition<2, NoThrow> {
   const intptr_t class_id_;
   const intptr_t token_pos_;
   const intptr_t element_count_;
+  Representation representation_;
 
   DISALLOW_COPY_AND_ASSIGN(LoadCodeUnitsInstr);
 };