[crankshaft] Re-add fast-case for string add left/right.

src/code-stubs.cc

Index: src/code-stubs.cc
diff --git a/src/code-stubs.cc b/src/code-stubs.cc
index 8bd7c5449bed939b598defdda68b26879fd117f9..06e7a65f2aa768bef39e2d4f8da35e4b293fe7e2 100644
--- a/src/code-stubs.cc
+++ b/src/code-stubs.cc
@@ -324,6 +324,12 @@ std::ostream& operator<<(std::ostream& os, const StringAddFlags& flags) {
       return os << "CheckRight";
     case STRING_ADD_CHECK_BOTH:
       return os << "CheckBoth";
+    case STRING_ADD_CONVERT_LEFT:
+      return os << "ConvertLeft";
+    case STRING_ADD_CONVERT_RIGHT:
+      return os << "ConvertRight";
+    case STRING_ADD_CONVERT:
+      break;
   }
   UNREACHABLE();
   return os;