[SVE] Eliminate calls to default-false VectorType::get() from SystemZ

Reviewers: efriedma, jnspaulsson, kmclaughlin, sdesmalen, samparker, uweigand

Reviewed By: uweigand

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80329