[SVE] Deprecate default false variant of VectorType::get

Reviewers: efriedma, fpetrogalli, kmclaughlin, huntergr

Reviewed By: fpetrogalli

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

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D80342