[Polly] Disable the parallel code generation in case of extension nodes

[gareevroman]

Summary

We can not perform the dependence analysis and, consequently, the parallel code generation in case the schedule tree contains extension nodes.

[grosser]

LGTM.

lib/CodeGen/IslAst.cpp

 IslAst::IslAst(Scop *Scop)
     : S(Scop), Root(nullptr), RunCondition(nullptr),
       Ctx(Scop->getSharedIslCtx()) {}
 
 void IslAst::init(const Dependences &D) {
   bool PerformParallelTest = PollyParallel || DetectParallel ||
                              PollyVectorizerChoice != VECTORIZER_NONE;
 
+  // We can not perform the dependence analysis and, consequently,
+  // the parallel code generation in case the schedule tree contains
+  // extension nodes.
+  auto *ScheduleTree = S->getScheduleTree();
+  PerformParallelTest =
+      PerformParallelTest && !S->containsExtensionNode(ScheduleTree);
+  isl_schedule_free(ScheduleTree);
+
   // Skip AST and code generation if there was no benefit achieved.
   if (!benefitsFromPolly(S, PerformParallelTest))
     return;
 
   isl_ctx *Ctx = S->getIslCtx();
   isl_options_set_ast_build_atomic_upper_bound(Ctx, true);
   isl_options_set_ast_build_detect_min_max(Ctx, true);
   isl_ast_build *Build;