[CodeGen] Remove calls to getVectorNumElements in DAGTypeLegalizer::SplitVecOp_EXTRACT_SUBVECTOR

In DAGTypeLegalizer::SplitVecOp_EXTRACT_SUBVECTOR I have replaced
calls to getVectorNumElements with getVectorMinNumElements, since
this code path works for both fixed and scalable vector types. For
scalable vectors the index will be multiplied by VSCALE.

Fixes warnings in this test:

  sve-sext-zext.ll

Differential revision: https://reviews.llvm.org/D83198