[AArch64][SVE] Add intrinsic for non-faulting loads

Summary:
This patch adds the llvm.aarch64.sve.ldnf1 intrinsic, plus
DAG combine rules for non-faulting loads and sign/zero extends

Reviewers: sdesmalen, efriedma, andwar, dancgr, mgudim, rengolin

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71698