Assume no annotations when visiting new domain (IslAst)

  Whe we build the IslAst we visit for nodes (in pre and post order) as well as
  user/domain nodes. As these two sets are non overlapping we do not need to
  check if we annotated a node earlier when we visit it.



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@214170 91177308-0d34-0410-b5e6-96231b3b80d8
[Refactor] Adjust some comments


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@214171 91177308-0d34-0410-b5e6-96231b3b80d8