[AArch64][SME] Add SVE2 psel, uclamp, sclamp and revd IR intrinsics

When the SME feature is enabled we also gain access to a few extra
SVE2 instructions. This patch adds LLVM IR intrinsics to make use
of these new instructions:

  @llvm.aarch64.sve.psel
  @llvm.aarch64.sve.revd
  @llvm.aarch64.sve.sclamp
  @llvm.aarch64.sve.uclamp

Differential Revision: https://reviews.llvm.org/D128332