ScheduleTree::ancestors: handle case where inputs are equal

The sequence of ancestors of a node within itself is simply
the empty sequence.  Handling this case in ancestors allows
special casing in activeDomainPoints to be removed.

Author: Sven Verdoolaege

tc/core/polyhedral/schedule_transforms.cc

@@ -111,9 +111,6 @@ isl::union_set activeDomainPoints(
   CHECK(domainElem) << "root must be a Domain node" << *root;
 
   auto domain = domainElem->domain_;
-  if (root == node) {
-    return domain;
-  }
 
   for (auto anc : node->ancestors(root)) {
     if (auto filterElem = anc->elemAsBase<ScheduleTreeElemFilter>()) {

tc/core/polyhedral/schedule_tree.cc

@@ -84,6 +84,9 @@ vector<size_t> positionRelativeToSubtree(
 vector<const ScheduleTree*> constAncestorsInSubTree(
     const ScheduleTree* relativeRoot,
     const ScheduleTree* target) {
+  if (relativeRoot == target) {
+    return vector<const ScheduleTree*>();
+  }
   vector<size_t> cp(positionRelativeToSubtree(relativeRoot, target));
   if (cp.size() == 0) {
     // Special case, this must be the root

tc/core/polyhedral/schedule_tree.h

@@ -262,6 +262,7 @@ struct ScheduleTree {
   // Returns the ancestors up to relativeRoot in a vector. The first element
   // of the result is relativeRoot, the last element of the result is the
   // father of the "this" ScheduleTree.
+  // If relativeRoot is equal to "this" ScheduleTree, then the result is empty.
   std::vector<ScheduleTree*> ancestors(ScheduleTree* relativeRoot);
   std::vector<const ScheduleTree*> ancestors(
       const ScheduleTree* relativeRoot) const;