[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.

llvm-svn: 213272