[Crankshaft] Don't add an instruction twice for %_StringGetLength.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index 268976af3ddb5416234e77dfe297fadcdfed6c25..d4b33ac5a9ecd61b268fa94ea6b4b85910ebc05f 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -1418,6 +1418,7 @@ class HGraphBuilder {
 
   HInstruction* AddLoadStringInstanceType(HValue* string);
   HInstruction* AddLoadStringLength(HValue* string);
+  HInstruction* BuildLoadStringLength(HValue* string);
   HStoreNamedField* AddStoreMapConstant(HValue* object, Handle<Map> map) {
     return Add<HStoreNamedField>(object, HObjectAccess::ForMap(),
                                  Add<HConstant>(map));