[SVE] Add new VectorType subclasses

Summary:
Introduce new types for fixed width and scalable vectors.

Does not remove getNumElements yet so as to not break code during transition
period.

Reviewers: deadalnix, efriedma, sdesmalen, craig.topper, huntergr

Reviewed By: sdesmalen

Subscribers: jholewinski, arsenm, jvesely, nhaehnle, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, csigg, arpith-jacob, mgester, lucyrfox, liufengdb, kerbowa, Joonsoo, grosul1, frgossen, lldb-commits, tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm, #lldb

Differential Revision: https://reviews.llvm.org/D77587