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

Reviewers: efriedma, sdesmalen, c-rhodes, hfinkel

Reviewed By: c-rhodes

Subscribers: wuzish, nemanjai, tschuett, hiraditya, kbarton, rkruppe, psnobl, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79821