[SLP] fix typo in debug string; NFC

Author: rotateright

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -1727,7 +1727,7 @@ class BoUpSLP {
         dbgs() << "NULL\n";
       dbgs() << "ReuseShuffleIndices: ";
       if (ReuseShuffleIndices.empty())
-        dbgs() << "Emtpy";
+        dbgs() << "Empty";
       else
         for (unsigned ReuseIdx : ReuseShuffleIndices)
           dbgs() << ReuseIdx << ", ";