[SVE] Remove calls to VectorType::getNumElements from CodeGen

Reviewers: efriedma, fpetrogalli, sdesmalen, RKSimon, arsenm

Reviewed By: RKSimon

Subscribers: wdng, tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82210