Mark phi as producing a smi value if it is dominated by SmiChecks over its operands.

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 4fbf480a64bb80d3626c968e81452ffc1a5b7b33..e9db19b8befc38ba5158803ab516fd55d94f40f0 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -62,7 +62,8 @@ class Value : public ZoneAllocated {
       : definition_(definition),
         next_use_(NULL),
         instruction_(NULL),
-        use_index_(-1) { }
+        use_index_(-1),
+        reaching_cid_(kIllegalCid) { }
 
   Definition* definition() const { return definition_; }
   void set_definition(Definition* definition) { definition_ = definition; }
@@ -107,12 +108,17 @@ class Value : public ZoneAllocated {
 
   bool Equals(Value* other) const;
 
+  void set_reaching_cid(intptr_t cid) { reaching_cid_ = cid; }
+  intptr_t reaching_cid() const { return reaching_cid_; }
+
  private:
   Definition* definition_;
   Value* next_use_;
   Instruction* instruction_;
   intptr_t use_index_;
 
+  intptr_t reaching_cid_;
+
   DISALLOW_COPY_AND_ASSIGN(Value);
 };