utils/abtest: Refactor and add bisection method

utils/abtest: Refactor and add bisection method

- Refactor/rewrite most of the code. Also make sure it passes
  pycodestyle/pyflakes now
- Add a new mode that performs bisection on the search space. This
  should be faster in the common case where there is only a small number
  of files or functions actually leading to failure.
  The previous sequential behavior can still be accessed via `--seq`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341679 91177308-0d34-0410-b5e6-96231b3b80d8