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

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
diff --git a/runtime/vm/flow_graph.h b/runtime/vm/flow_graph.h
index 5d67a5e9ee2ac3da0a24da0c6f50c5b2daaeffb3..29952be4228cba19112753ce1a40a02a9a3eec1f 100644
--- a/runtime/vm/flow_graph.h
+++ b/runtime/vm/flow_graph.h
@@ -81,6 +81,8 @@ class FlowGraph : public ZoneAllocated {
     return BlockIterator(postorder());
   }
 
+  intptr_t current_ssa_temp_index() const { return current_ssa_temp_index_; }
+
   intptr_t max_virtual_register_number() const {
     return current_ssa_temp_index();
   }
@@ -141,8 +143,6 @@ class FlowGraph : public ZoneAllocated {
 
   void MarkLivePhis(GrowableArray<PhiInstr*>* live_phis);
 
-  intptr_t current_ssa_temp_index() const { return current_ssa_temp_index_; }
-
   // DiscoverBlocks computes parent_ and assigned_vars_ which are then used
   // if/when computing SSA.
   GrowableArray<intptr_t> parent_;