// Marks 'resource' as written.
  void SetPotentiallyWritten(Value resource);

  struct ResourceInfo {
    bool potentially_written = false;
  };
  // Key: Resource Value's
  // Value: Information we know about that Value.
  // Note that these Value's are in general in different functions.
  DenseMap<Value, ResourceInfo> resource_infos_;
  // The set of regions we already discovered.
  DenseSet<Region*> discovered_;