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

Reviewers: efriedma, fhahn, reames, kmclaughlin, sdesmalen

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82243