Improve polymorphic loads on single slots.

src/ia32/lithium-ia32.h

Index: src/ia32/lithium-ia32.h
diff --git a/src/ia32/lithium-ia32.h b/src/ia32/lithium-ia32.h
index bf8dbc636dc0c073cca82382bb4b9b3d6f3e649c..9d8cdfd87e17c5fead7fe9652a52ff7d43ffde30 100644
--- a/src/ia32/lithium-ia32.h
+++ b/src/ia32/lithium-ia32.h
@@ -66,6 +66,7 @@ class LCodeGen;
   V(CheckFunction)                              \
   V(CheckInstanceType)                          \
   V(CheckMap)                                   \
+  V(CheckMapSet)                                \
   V(CheckNonSmi)                                \
   V(CheckPrototypeMaps)                         \
   V(CheckSmi)                                   \
@@ -1925,6 +1926,17 @@ class LCheckMap: public LTemplateInstruction<0, 1, 0> {
 };
 
 
+class LCheckMapSet: public LTemplateInstruction<0, 1, 0> {
+ public:
+  explicit LCheckMapSet(LOperand* value) {
+    inputs_[0] = value;
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(CheckMapSet, "check-map-set")
+  DECLARE_HYDROGEN_ACCESSOR(CheckMapSet)
+};
+
+
 class LCheckPrototypeMaps: public LTemplateInstruction<0, 0, 1> {
  public:
   explicit LCheckPrototypeMaps(LOperand* temp)  {