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

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 4389bfe78ab391c44a24a8ab36874e57968046cf..389a943ee66f274c60a112769b60a9a521e16e39 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -6840,13 +6840,18 @@ HInstruction* HGraphBuilder::AddLoadStringInstanceType(HValue* string) {
 
 
 HInstruction* HGraphBuilder::AddLoadStringLength(HValue* string) {
+  return AddInstruction(BuildLoadStringLength(string));
+}
+
+
+HInstruction* HGraphBuilder::BuildLoadStringLength(HValue* string) {
   if (string->IsConstant()) {
     HConstant* c_string = HConstant::cast(string);
     if (c_string->HasStringValue()) {
-      return Add<HConstant>(c_string->StringValue()->length());
+      return New<HConstant>(c_string->StringValue()->length());
     }
   }
-  return Add<HLoadNamedField>(string, nullptr,
+  return New<HLoadNamedField>(string, nullptr,
                               HObjectAccess::ForStringLength());
 }
 
@@ -11879,7 +11884,7 @@ void HOptimizedGraphBuilder::GenerateStringGetLength(CallRuntime* call) {
   DCHECK(call->arguments()->length() == 1);
   CHECK_ALIVE(VisitForValue(call->arguments()->at(0)));
   HValue* string = Pop();
-  HInstruction* result = AddLoadStringLength(string);
+  HInstruction* result = BuildLoadStringLength(string);
   return ast_context()->ReturnInstruction(result, call->id());
 }