[SVE] Replace deprecated call in changeVectorElementTypeToInteger

Summary:
Replace getVectorNumElements with getVectorElementCount;
gets rid of the warnings in several tests

Reviewers: sdesmalen, kmclaughlin, dancgr, efriedma, each, andwar, rengolin

Reviewed By: efriedma

Subscribers: tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80746