[AArch64][SVE] Add DAG combine rules for gather loads and sext/zext

Summary:
These changes allow us to support sign-extending gather loads with the
exisiting intrinsics (i.e. @llvm.aarch64.sve.ld1.gather.*).

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

Reviewed By: sdesmalen

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

Tags: #llvm

Differential revision: https://reviews.llvm.org/D70812