[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

llvm-svn: 213792