toIslScheduleTree: swap the order of non-core filters below extension

When converting TC ScheduleTree into isl schedule_tree, children of an
extension nodes are introduced progressively, starting with "core"
children, i.e. those parts of the extension children that relate to the
points present in the root domain.  Other children are added
before(after) the core children using node.graft_before(node.graft_after)
on the fist(last) core child.  These functions insert the new sibling
subtree before(after) the node and return the node itself.  Therefore,
when calling these functions multiple times, the order in which siblings
are added must be inverse to their expected order of appearance.
Existing implementation used the direct order, revert it.

Author: ftynse

src/core/polyhedral/schedule_isl_conversion.cc

@@ -156,12 +156,12 @@ isl::schedule_node insertExtension(
     }
     node = node.ancestor(node.get_tree_depth() - depth0);
   }
-  for (auto i = corePos[0]; i >= 1; --i) {
-    auto graft = graftFromFilterSubtree(child->child({i - 1}), extension);
+  for (size_t i = 0; i < corePos[0]; ++i) {
+    auto graft = graftFromFilterSubtree(child->child({i}), extension);
     node = node.graft_before(graft);
   }
-  for (auto i = corePos.back() + 1; i < child->numChildren(); ++i) {
-    auto graft = graftFromFilterSubtree(child->child({i}), extension);
+  for (auto i = child->numChildren(); i > corePos.back() + 1; --i) {
+    auto graft = graftFromFilterSubtree(child->child({i - 1}), extension);
     node = node.graft_after(graft);
   }
   node = node.ancestor(node.get_tree_depth() - depth0);