[Vectorize] Migrate away from PointerUnion::dyn_cast (NFC)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect InVectors.front() and P to be nonnull.

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -10354,7 +10354,7 @@ class BoUpSLP::ShuffleCostEstimator : public BaseShuffleAnalysis {
     SameNodesEstimated = false;
     if (!E2 && InVectors.size() == 1) {
       unsigned VF = E1.getVectorFactor();
-      if (Value *V1 = InVectors.front().dyn_cast<Value *>()) {
+      if (Value *V1 = dyn_cast<Value *>(InVectors.front())) {
         VF = std::max(VF,
                       cast<FixedVectorType>(V1->getType())->getNumElements());
       } else {
@@ -10370,7 +10370,7 @@ class BoUpSLP::ShuffleCostEstimator : public BaseShuffleAnalysis {
       auto P = InVectors.front();
       Cost += createShuffle(&E1, E2, Mask);
       unsigned VF = Mask.size();
-      if (Value *V1 = P.dyn_cast<Value *>()) {
+      if (Value *V1 = dyn_cast<Value *>(P)) {
         VF = std::max(VF,
                       getNumElements(V1->getType()));
       } else {