Improve polymorphic loads on single slots.

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index 758701652049b7251446d473ef4064c741e2fca5..d352076c894fdebf34daec34175970a8481f10fe 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -1795,6 +1795,13 @@ LInstruction* LChunkBuilder::DoCheckMap(HCheckMap* instr) {
 }
 
 
+LInstruction* LChunkBuilder::DoCheckMapSet(HCheckMapSet* instr) {
+  LOperand* value = UseRegisterAtStart(instr->value());
+  LCheckMapSet* result = new(zone()) LCheckMapSet(value);
+  return AssignEnvironment(result);
+}
+
+
 LInstruction* LChunkBuilder::DoClampToUint8(HClampToUint8* instr) {
   HValue* value = instr->value();
   Representation input_rep = value->representation();