[SLP][NFC]Use MutableArrayRef instead of SmallVectorImpl&, NFC.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -858,7 +858,7 @@ static void addMask(SmallVectorImpl<int> &Mask, ArrayRef<int> SubMask,
 /// values 3 and 7 respectively:
 /// before:  6 9 5 4 9 2 1 0
 /// after:   6 3 5 4 7 2 1 0
-static void fixupOrderingIndices(SmallVectorImpl<unsigned> &Order) {
+static void fixupOrderingIndices(MutableArrayRef<unsigned> Order) {
   const unsigned Sz = Order.size();
   SmallBitVector UnusedIndices(Sz, /*t=*/true);
   SmallBitVector MaskedIndices(Sz);