[SVE][CodeGen] Legalisation of vsetcc with scalable types

Summary: Changes SplitVecOp_VSETCC to use getVectorElementCount()

Reviewers: sdesmalen, efriedma, dancgr

Reviewed By: efriedma

Subscribers: david-arm, tschuett, hiraditya, rkruppe, psnobl, huihuiz, cfe-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79167