TensorReferenceGroup::makeSingleton: explicitly drop reference tag

The group approximation should not have a reference tag since
it may cover multiple references.  The tag currently gets removed
implicitly because outputRanges flattens the domain space.
Explicitly drop the tags before calling outputRanges.
Note that the call to outputRanges occurs a couple of lines down
from the changes.

Author: Sven Verdoolaege

tc/core/polyhedral/memory_promotion.cc

@@ -117,8 +117,9 @@ std::unique_ptr<TensorReferenceGroup> TensorReferenceGroup::makeSingleton(
   auto ref = std::unique_ptr<TensorReference>(new TensorReference);
   auto refId = scopedAccess.get_space().domain().unwrap().get_tuple_id(
       isl::dim_type::out);
+  scopedAccess = scopedAccess.domain_factor_domain();
   ref->originalAccess = originalAccess.domain_factor_domain();
-  ref->scopedAccess = scopedAccess.domain_factor_domain();
+  ref->scopedAccess = scopedAccess;
   ref->type = type;
   ref->refId = refId;
   auto group = std::unique_ptr<TensorReferenceGroup>(new TensorReferenceGroup);