[AArch64] Replace SmallSet with SmallPtrSet (NFC) (#154264)

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

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -7498,7 +7498,7 @@ static bool getGatherLanePattern(MachineInstr &Root,
   // limit.
   auto MBBItr = Root.getIterator();
   unsigned RemainingSteps = GatherOptSearchLimit;
-  SmallSet<const MachineInstr *, 16> RemainingLoadInstrs;
+  SmallPtrSet<const MachineInstr *, 16> RemainingLoadInstrs;
   RemainingLoadInstrs.insert(LoadInstrs.begin(), LoadInstrs.end());
   const MachineBasicBlock *MBB = Root.getParent();