[Scalar] Remove an unused variable (llvm#105767)

The last use was removed by:

  commit 89fe570
  Author: Philip Reames <listmail@philipreames.com>
  Date:   Tue May 12 23:39:23 2015 +0000

llvm/lib/Transforms/Scalar/PlaceSafepoints.cpp

@@ -342,7 +342,6 @@ bool PlaceSafepointsPass::runImpl(Function &F, const TargetLibraryInfo &TLI) {
         // The split loop structure here is so that we only need to recalculate
         // the dominator tree once.  Alternatively, we could just keep it up to
         // date and use a more natural merged loop.
-        SetVector<BasicBlock *> SplitBackedges;
         for (BasicBlock *Header : Headers) {
           BasicBlock *NewBB = SplitEdge(Term->getParent(), Header, &DT);
           PollsNeeded.push_back(NewBB->getTerminator());