[Refactor] Remove containsLoop to find innermost loops

  Use the fact that if we visit a for node first in pre and next in post order
  we know we did not visit any children, thus we found an innermost loop.

  + Test case for an innermost loop with a conditional inside



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@213870 91177308-0d34-0410-b5e6-96231b3b80d8