[AArch64][SVE] Add intrinsics for gather loads with 64-bit offsets

This patch adds the following intrinsics for gather loads with 64-bit offsets:
      * @llvm.aarch64.sve.ld1.gather (unscaled offset)
      * @llvm.aarch64.sve.ld1.gather.index (scaled offset)

These intrinsics map 1-1 to the following AArch64 instructions respectively (examples for half-words):
      * ld1h { z0.d }, p0/z, [x0, z0.d]
      * ld1h { z0.d }, p0/z, [x0, z0.d, lsl #1]

Committing on behalf of Andrzej Warzynski (andwar)

Reviewers: sdesmalen, huntergr, rovka, mgudim, dancgr, rengolin, efriedma

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70542