[SVE] Change return type of getNumElements to unsigned

Reviewers: efriedma, sdesmalen, craig.topper, dexonsmith

Reviewed By: efriedma, sdesmalen

Subscribers: tschuett, hiraditya, rkruppe, psnobl, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, grosul1, frgossen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77763