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

Reviewers: efriedma, greened, c-rhodes, david-arm, dmgreen

Reviewed By: dmgreen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82216