Don't CheckSmi when type is Smi.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 5c573feb198c9cddc227186db482388bad94bdb7..cd46747fb0234d67e31d39bf1fed1eefff7e1944 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1129,8 +1129,10 @@ HInstruction* HGraphBuilder::BuildFastElementAccess(
     switch (elements_kind) {
       case FAST_SMI_ELEMENTS:
       case FAST_HOLEY_SMI_ELEMENTS:
-        // Smi-only arrays need a smi check.
-        AddInstruction(new(zone) HCheckSmi(val));
+        if (!val->type().IsSmi()) {
+          // Smi-only arrays need a smi check.
+          AddInstruction(new(zone) HCheckSmi(val));
+        }
         // Fall through.
       case FAST_ELEMENTS:
       case FAST_HOLEY_ELEMENTS:
@@ -6934,9 +6936,11 @@ void HOptimizedGraphBuilder::VisitArrayLiteral(ArrayLiteral* expr) {
     switch (boilerplate_elements_kind) {
       case FAST_SMI_ELEMENTS:
       case FAST_HOLEY_SMI_ELEMENTS:
-        // Smi-only arrays need a smi check.
-        AddInstruction(new(zone()) HCheckSmi(value));
-        // Fall through.
+        if (!value->type().IsSmi()) {
+          // Smi-only arrays need a smi check.
+          AddInstruction(new(zone()) HCheckSmi(value));
+          // Fall through.
+        }
       case FAST_ELEMENTS:
       case FAST_HOLEY_ELEMENTS:
       case FAST_DOUBLE_ELEMENTS: