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

Reviewers: efriedma, david-arm, c-rhodes, jnspaulsson

Reviewed By: david-arm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79824