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

Reviewers: efriedma, kmclaughlin, sdesmalen, kparzysz

Reviewed By: kparzysz

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79819