[SVE] Remove calls to VectorType::getNumElements from AArch64

Reviewers: efriedma, paquette, david-arm, kmclaughlin

Reviewed By: david-arm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82214