[CodeGen] Fix wrong use of getVectorNumElements() in DAGTypeLegalizer::SplitVecRes_ExtendOp

In DAGTypeLegalizer::SplitVecRes_ExtendOp I have replaced an invalid
call to getVectorNumElements() with a call to getVectorMinNumElements(),
since the code path works for both fixed and scalable vectors.

This fixes up a warning in the following test:

  sve-sext-zext.ll

Differential Revision: https://reviews.llvm.org/D83197

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

@@ -1778,8 +1778,7 @@ void DAGTypeLegalizer::SplitVecRes_ExtendOp(SDNode *N, SDValue &Lo,
   // more effectively move in the right direction and prevent falling down
   // to scalarization in many cases due to the input vector being split too
   // far.
-  unsigned NumElements = SrcVT.getVectorNumElements();
-  if ((NumElements & 1) == 0 &&
+  if ((SrcVT.getVectorMinNumElements() & 1) == 0 &&
       SrcVT.getSizeInBits() * 2 < DestVT.getSizeInBits()) {
     LLVMContext &Ctx = *DAG.getContext();
     EVT NewSrcVT = SrcVT.widenIntegerVectorElementType(Ctx);