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

Reviewers: efriedma, jdoerfert, sdesmalen, kmclaughlin

Reviewed By: efriedma, jdoerfert

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82219