[CodeGen] Replace calls to getVectorNumElements() in SelectionDAG::SplitVector

This patch replaces some invalid calls to getVectorNumElements() with calls
to getVectorMinNumElements() instead, since the code paths changed in this
patch work for both fixed and scalable vector types.

Fixes warnings in this test:

  sve-sext-zext.ll

Differential Revision: https://reviews.llvm.org/D83203