[Refactor] Remove unecessary check and function

  + Perform the parallelism check on the innermost loop only once.
  + Inline the markOpenmpParallel function.
  + Rename all IslAstUserPayload * into Payload to make it consistent.



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@214448 91177308-0d34-0410-b5e6-96231b3b80d8