message: Make sure RegAlloc runs at end if no config given
  Summary: Fixes
diff: @@ -134,7 +134,7 @@ Json::Value default_config() {
  "ReBindRefsPass", "BridgePass", "SynthPass",
  "FinalInlinePass", "DelSuperPass", "SingleImplPass",
  "SimpleInlinePass", "StaticReloPass", "RemoveEmptyClassesPass",
  - "ShortenSrcStringsPass",
  + "ShortenSrcStringsPass", "RegAllocPass",
  };
  std::istringstream temp_json("{\"redex\":{\"passes\":[]}}");
  Json::Value cfg;
  