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

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index 01da2c52fffd4f1b6802210bdfb25a9f8b32da39..a359f8cf1e6a6a938a008346c3ba1557484e08fe 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -203,6 +203,9 @@ static bool CompileParsedFunctionHelper(const ParsedFunction& parsed_function,
         // Verify that the use lists are still valid.
         DEBUG_ASSERT(flow_graph->ValidateUseLists());
 
+        // Propagate sminess from CheckSmi to phis.
+        optimizer.PropagateSminess();
+
         // Do optimizations that depend on the propagated type information.
         optimizer.OptimizeComputations();