[Refactor] IslAst and payload struct

  + Renamed context into build when it's the isl_ast_build
  + Use the IslAstInfo functions to extract the schedule of a node
  + Use the IslAstInfo functions to extract the build/context of a node
  + Move the payload struct into the IslAstInfo class
  + Use a constructor and destructor (also new and delete) to
    allocate/initialize the payload struct


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@213792 91177308-0d34-0410-b5e6-96231b3b80d8