[mlir] Replace SmallSet with SmallPtrSet (NFC) (#154265)

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 30 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.

Author: kazutakahirata

mlir/lib/Dialect/Async/Transforms/AsyncRuntimeRefCounting.cpp

@@ -248,7 +248,7 @@ LogicalResult AsyncRuntimeRefCountingPass::addDropRefAfterLastUse(Value value) {
   Region *definingRegion = value.getParentRegion();
 
   // Last users of the `value` inside all blocks where the value dies.
-  llvm::SmallSet<Operation *, 4> lastUsers;
+  llvm::SmallPtrSet<Operation *, 4> lastUsers;
 
   // Find blocks in the `definingRegion` that have users of the `value` (if
   // there are multiple users in the block, which one will be selected is