[polly] Replace SmallSet with SmallPtrSet (NFC) (#154367)

This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>.  Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:

  template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};

We only have 10 instances that rely on this "redirection". Since the
redirection doesn't improve readability, this patch replaces SmallSet
with SmallPtrSet for pointer element types.

I'm planning to remove the redirection eventually.

polly/lib/Analysis/ScopBuilder.cpp

@@ -689,7 +689,7 @@ isl::set ScopBuilder::getPredecessorDomainConstraints(BasicBlock *BB,
 
   // Set of regions of which the entry block domain has been propagated to BB.
   // all predecessors inside any of the regions can be skipped.
-  SmallSet<Region *, 8> PropagatedRegions;
+  SmallPtrSet<Region *, 8> PropagatedRegions;
 
   for (auto *PredBB : predecessors(BB)) {
     // Skip backedges.