[AArch64][SVE] Add reg+imm addressing mode for unpredicated loads

Reviewers: efriedma, sdesmalen, david-arm

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82893