[CodeGen] Fix warning in visitShuffleVector

[david-arm]

Summary

Make sure we only ask for the number of elements after we've
bailed out for scalable vectors.

[fpetrogalli]

LGTM, thank you!

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

void SelectionDAGBuilder::visitShuffleVector(const User &I) {
   if (auto *SVI = dyn_cast<ShuffleVectorInst>(&I))
     Mask = SVI->getShuffleMask();
   else
     Mask = cast<ConstantExpr>(I).getShuffleMask();
   SDLoc DL = getCurSDLoc();
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   EVT VT = TLI.getValueType(DAG.getDataLayout(), I.getType());
   EVT SrcVT = Src1.getValueType();
-  unsigned SrcNumElts = SrcVT.getVectorNumElements();
 
   if (all_of(Mask, [](int Elem) { return Elem == 0; }) &&
       VT.isScalableVector()) {
     // Canonical splat form of first element of first input vector.
     SDValue FirstElt =
         DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, SrcVT.getScalarType(), Src1,
                     DAG.getVectorIdxConstant(0, DL));
     setValue(&I, DAG.getNode(ISD::SPLAT_VECTOR, DL, VT, FirstElt));
     return;
   }
 
   // For now, we only handle splats for scalable vectors.
   // The DAGCombiner will perform a BUILD_VECTOR -> SPLAT_VECTOR transformation
   // for targets that support a SPLAT_VECTOR for non-scalable vector types.
   assert(!VT.isScalableVector() && "Unsupported scalable vector shuffle");
 
+  unsigned SrcNumElts = SrcVT.getVectorNumElements();
   unsigned MaskNumElts = Mask.size();
 
   if (SrcNumElts == MaskNumElts) {
     setValue(&I, DAG.getVectorShuffle(VT, DL, Src1, Src2, Mask));
     return;
   }
 
   // Normalize the shuffle vector since mask and vector length don't match.