[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