The following if-condition is always true accroding to sole caller, so change it to assert `is_old || is_humongous`.
if (tracker->needs_scan_for_rebuild(r)) {
_top_at_rebuild_starts[region] = r->top();
} else {
// Leave TARS at null.
}
if (tracker->needs_scan_for_rebuild(r)) {
_top_at_rebuild_starts[region] = r->top();
} else {
// Leave TARS at null.
}