introduce insertTopLevelEmptyBand

This function inserts an empty band below the root of the tree or below
the top-level context if it is present.  It will be used in the
following commit.

Author: ftynse

tc/core/polyhedral/schedule_transforms.cc

@@ -275,6 +275,15 @@ ScheduleTree* bandScale(ScheduleTree* tree, const vector<size_t>& scales) {
   return tree;
 }
 
+ScheduleTree* insertTopLevelEmptyBand(ScheduleTree* root) {
+  auto node = root;
+  if (node->numChildren() > 0 &&
+      node->child({0})->elemAs<detail::ScheduleTreeElemContext>()) {
+    node = node->child({0});
+  }
+  return insertNodeBelow(node, ScheduleTree::makeEmptyBand(root));
+}
+
 void updateTopLevelContext(detail::ScheduleTree* root, isl::set context) {
   if (!matchOne(tc::polyhedral::domain(tc::polyhedral::context(any())), root)) {
     root->appendChild(ScheduleTree::makeContext(

tc/core/polyhedral/schedule_transforms.h

@@ -102,6 +102,10 @@ detail::ScheduleTree* bandScale(
     detail::ScheduleTree* tree,
     const std::vector<size_t>& scales);
 
+// Insert an empty band node below "root" or below the only child of "root" if
+// the child is a context node.
+detail::ScheduleTree* insertTopLevelEmptyBand(detail::ScheduleTree* root);
+
 // Update the top-level context node by intersecting it with "context".  The
 // top-level context node must be located directly under the root of the tree.
 // If there is no such node, insert one with universe context first.