Remove unused flag polly-allow-non-scev-backedge-taken-count

  Drop an unused flag polly-allow-non-scev-backedge-taken-count and also
  its occurrences from the tests.

Contributed-by: Chris Jenneisch <chrisj@codeaurora.org>

Differential Revision: http://reviews.llvm.org/D13400

llvm-svn: 249675