[SVE] Eliminate calls to default-false VectorType::get() from AArch64

Reviewers: efriedma, c-rhodes, david-arm, mcrosier, t.p.northover

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80327