[SLP] Fix placement of debug statement (NFC)

[mzolotukhin]

LGTM, thanks!

lib/Transforms/Vectorize/SLPVectorizer.cpp

for (unsigned N = 0; N < VectorizableTree.size(); ++N) {
     if (!Inst)
       continue;
 
     if (!PrevInst) {
       PrevInst = Inst;
       continue;
     }
 
+    // Update LiveValues.
+    LiveValues.erase(PrevInst);
+    for (auto &J : PrevInst->operands()) {
+      if (isa<Instruction>(&*J) && ScalarToTreeEntry.count(&*J))
+        LiveValues.insert(cast<Instruction>(&*J));
+    }
+
     DEBUG(
       dbgs() << "SLP: #LV: " << LiveValues.size();
       for (auto *X : LiveValues)
         dbgs() << " " << X->getName();
       dbgs() << ", Looking at ";
       Inst->dump();
       );
 
-    // Update LiveValues.
-    LiveValues.erase(PrevInst);
-    for (auto &J : PrevInst->operands()) {
-      if (isa<Instruction>(&*J) && ScalarToTreeEntry.count(&*J))
-        LiveValues.insert(cast<Instruction>(&*J));
-    }
-
     // Now find the sequence of instructions between PrevInst and Inst.
     BasicBlock::reverse_iterator InstIt(Inst->getIterator()),
         PrevInstIt(PrevInst->getIterator());
     --PrevInstIt;
     while (InstIt != PrevInstIt) {
       if (PrevInstIt == PrevInst->getParent()->rend()) {
         PrevInstIt = Inst->getParent()->rbegin();
         continue;