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

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 069528f16caff523eacd1fd4959aad9043416f87..eedceed52beac0d56b0996a8474524bf47e1f25a 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -742,7 +742,10 @@ RawAbstractType* Value::CompileType() const {
 
 
 intptr_t Value::ResultCid() const {
-  return definition()->GetPropagatedCid();
+  if (reaching_cid() == kIllegalCid) {
+    return definition()->GetPropagatedCid();
+  }
+  return reaching_cid();
 }