[CodeGen] Replace calls to getVectorNumElements() in DAGTypeLegalizer::SetSplitVector

In DAGTypeLegalizer::SetSplitVector I have changed calls in the assert
from getVectorNumElements() to getVectorElementCount(), since this
code path works for both fixed and scalable vectors.

This fixes up one warning in the test:

  sve-sext-zext.ll

Differential Revision: https://reviews.llvm.org/D83196