[Fix] OpenMP parallel loop detection for the isl backend

  There was a bug in the IslAst which caused that no more outermost
  parallel loops were detected/checked after a parallel outermost loop
  of depth 1.

  + Test case attached


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@217452 91177308-0d34-0410-b5e6-96231b3b80d8