We can only merge nested loops if that does not destroy
attribute information. While it might be possible to merge
loops with attributes in some cases, for now we just disallow
merging if any of the two loops have additional attributes.
This change prevents the canonicalizer from destroying
information added with greadilyMapParallelSCFToGPU.
Checking whether attr is not in ParallelOp::getAttributeNames() might be slightly better.