[SVE] Remove calls to VectorType::getNumElements from X86

Reviewers: efriedma, RKSimon, craig.topper, fpetrogalli, c-rhodes

Reviewed By: RKSimon

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82508