[SVE] Remove usages of VectorType::getNumElements() from ARM

Reviewers: efriedma, fpetrogalli, kmclaughlin, grosbach, dmgreen

Reviewed By: dmgreen

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, dmgreen, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79816