[Aarch64][SVE] Add intrinsics for scatter stores

Summary:
This patch adds the following SVE intrinsics for scatter stores:
* 64-bit offsets:
  * @llvm.aarch64.sve.st1.scatter (unscaled)
  * @llvm.aarch64.sve.st1.scatter.index (scaled)
* 32-bit unscaled offsets:
  * @llvm.aarch64.sve.st1.scatter.uxtw (zero-extended offset)
  * @llvm.aarch64.sve.st1.scatter.sxtw (sign-extended-offset)
* 32-bit scaled offsets:
  * @llvm.aarch64.sve.st1.scatter.uxtw.index (zero-extended offset)
  * @llvm.aarch64.sve.st1.scatter.sxtw.index (sign-extended offset)
* vector base + immediate:
  * @llvm.aarch64.sve.st1.scatter.imm

Reviewers: rengolin, efriedma, sdesmalen

Reviewed By: efriedma, sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71074