MappedScop::detectReductions: do not detect reduction if block is of size 1

Test TcCudaMapper2DReductionStressTest.ThreadIdy1 explicitly checks
that no reduction is detected if the block is of size 1 in the x direction.
This is only the case because of some artifact of the current
reduction detection mechanism.  In particular, after specialization,
the schedule band members on a domain of length 0 are replaced
by constants and therefore no longer contain reduction dimensions and
are therefore not detected as reduction band members.

The reduction detection based on the polyhedral representation
of reductions does not suffer from such artifacts and will
therefore, correctly, identify this case as a reduction.
Since this is apparently undesirable, an explicit check needs
to be introduced that disables reduction detection if the block
is of size 1 in the x direction.

Author: Sven Verdoolaege

tc/core/polyhedral/cuda/mapped_scop.cc

@@ -226,6 +226,11 @@ bool separatedOut(
 } // namespace
 
 bool MappedScop::detectReductions(detail::ScheduleTree* tree) {
+  // Do not bother with reductions if block is of size 1 in the x direction.
+  if (numThreads.view.size() == 0 || numThreads.view[0] == 1) {
+    return false;
+  }
+
   bool found = false;
   for (auto c : tree->children()) {
     found |= detectReductions(c);