[Refactor] Move code out of the IslAst header

  Offer the static functions to extract information out of an IslAst for node
  as members of IslAstInfo not as top level entities.

  + Refactor common code
  + Add isParallel and isReductionParallel
  + Rename IslAstUser to IslAstUserPayload to make it clear this is just a (or
    the) payload struct.


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@213272 91177308-0d34-0410-b5e6-96231b3b80d8