[SVE] Remove bad calls to VectorType::getNumElements() from PowerPC

llvm/lib/Target/PowerPC/PPCTargetTransformInfo.cpp

PPCTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
   case Intrinsic::ppc_altivec_vperm:
     // Turn vperm(V1,V2,mask) -> shuffle(V1,V2,mask) if mask is a constant.
     // Note that ppc_altivec_vperm has a big-endian bias, so when creating
     // a vectorshuffle for little endian, we must undo the transformation
     // performed on vec_perm in altivec.h.  That is, we must complement
     // the permutation mask with respect to 31 and reverse the order of
     // V1 and V2.
     if (Constant *Mask = dyn_cast<Constant>(II.getArgOperand(2))) {
-      assert(cast<VectorType>(Mask->getType())->getNumElements() == 16 &&
+      assert(cast<FixedVectorType>(Mask->getType())->getNumElements() == 16 &&
              "Bad type for intrinsic!");
 
       // Check that all of the elements are integer constants or undefs.
       bool AllEltsOk = true;
       for (unsigned i = 0; i != 16; ++i) {
         Constant *Elt = Mask->getAggregateElement(i);
         if (!Elt || !(isa<ConstantInt>(Elt) || isa<UndefValue>(Elt))) {
           AllEltsOk = false;