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

Reviewers: efriedma, arsenm, david-arm, fpetrogalli

Reviewed By: efriedma

Subscribers: dmgreen, arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, tschuett, hiraditya, rkruppe, psnobl, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79807