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

Reviewers: efriedma, bkramer, kmclaughlin, sdesmalen

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82212