[Transforms] Include llvm/ADT/SmallSet.h (NFC)

This patch adds #include "llvm/ADT/SmallSet.h" to a couple of files
that are relying on transitive includes of SmallSet.h.  It in turn
unblocks the removal of unnecessary includes of llvm/ADT/SmallSet.h in
several other files.

Author: kazutakahirata

llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp

@@ -19,6 +19,7 @@
 
 #include "llvm/Transforms/Scalar/LowerMatrixIntrinsics.h"
 #include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/DomTreeUpdater.h"

llvm/lib/Transforms/Utils/CanonicalizeFreezeInLoops.cpp

@@ -30,6 +30,7 @@
 
 #include "llvm/Transforms/Utils/CanonicalizeFreezeInLoops.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Analysis/IVDescriptors.h"
 #include "llvm/Analysis/LoopAnalysisManager.h"