insertCopiesUnder: make groupId argument mandatory

The default null value of the groupId in insertCopiesUnder was left over
during a refactoring in the prehistory.  The function checks if it is
not null early, without any side effects before the check.  Remove the
default value of the argument and the check.  The group id is now
mandatory, and it is ensured by a compile-time rather than by a runtime
check.

Author: ftynse

tc/core/polyhedral/memory_promotion.cc

@@ -475,9 +475,6 @@ ScheduleTree* insertCopiesUnder(
   // Take the set of all tensor elements.
   auto tensorElements = tensorElementsSet(scop, tensorId);
 
-  if (groupId.is_null()) {
-    throw promotion::GroupingError("expected group id");
-  }
   auto promotion =
       isl::map(group.promotion()).set_tuple_id(isl::dim_type::out, groupId);
   auto promotionSpace = promotion.get_space();

tc/core/polyhedral/memory_promotion.h

@@ -214,6 +214,6 @@ detail::ScheduleTree* insertCopiesUnder(
     detail::ScheduleTree* tree,
     const TensorReferenceGroup& group,
     isl::id tensorId,
-    isl::id groupId = isl::id());
+    isl::id groupId);
 } // namespace polyhedral
 } // namespace tc