Merge changes I018652c9,I51c6fe63

* changes:
  Prohibit recursive models
  Implement control flow operation validation
tree: 0b1e1f7fecd7acbb861d68f66f614f3cfb660989
  1. nn/
  2. .clang-format
  3. CleanSpec.mk
  4. CPPLINT.cfg
  5. OWNERS
  6. PREUPLOAD.cfg