[Attributor][FIX] Undo 16188f9d until SCC iterator bug is fixed

The buildbot
  http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win
shows some strange SCC iterator bug since 16188f9d which we need to
investigate. This patch should remove the part of 16188f9d that could
have exposed the problem.