[AArch64][SVE] Add SVE index intrinsic

Summary:
Implements the @llvm.aarch64.sve.index intrinsic, which
takes a scalar base and step value.

This patch also adds the printSImm function to AArch64InstPrinter
to ensure that immediates of type i8 & i16 are printed correctly.

Reviewers: sdesmalen, andwar, efriedma, dancgr, cameron.mcinally, rengolin

Reviewed By: cameron.mcinally

Subscribers: tatyana-krasnukha, tschuett, kristof.beyls, hiraditya, rkruppe, arphaman, psnobl, cfe-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74550