[SVE] Remove calls to isScalable from Hexagon

[ctetreau]

While immediate feedback would be greatly appreciated, this commit won't actually build until D77587 is merged. I'll ping the thread when this happens if this commit hasn't been accepted yet.

llvm/lib/Target/Hexagon/HexagonTargetTransformInfo.cpp

 static const unsigned FloatFactor = 4;
 
 bool HexagonTTIImpl::useHVX() const {
   return ST.useHVXOps() && HexagonAutoHVX;
 }
 
 bool HexagonTTIImpl::isTypeForHVX(Type *VecTy) const {
   assert(VecTy->isVectorTy());
-  if (cast<VectorType>(VecTy)->isScalable())
+  if (isa<ScalableVectorType>(VecTy))
     return false;
   // Avoid types like <2 x i32*>.
   if (!cast<VectorType>(VecTy)->getElementType()->isIntegerTy())
     return false;
   EVT VecVT = EVT::getEVT(VecTy);
   if (!VecVT.isSimple() || VecVT.getSizeInBits() <= 64)
     return false;
   if (ST.isHVXVectorType(VecVT.getSimpleVT()))