[SVE] Remove calls to isScalable from AARCH64

Reviewers: efriedma, sdesmalen, t.p.northover, mcrosier

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77758