[AArch64][SVE] Add intrnisics for saturating scalar arithmetic

Summary:
The following intrnisics are added:
  * @llvm.aarch64.sve.sqdec{b|h|w|d|p}
  * @llvm.aarch64.sve.sqinc{b|h|w|d|p}
  * @llvm.aarch64.sve.uqdec{b|h|w|d|p}
  * @llvm.aarch64.sve.uqinc{b|h|w|d|p}

For every intrnisic there a scalar variants (with n32 or n64 suffix) and
vector variants (no suffix).

Reviewers: sdesmalen, rengolin, efriedma

Reviewed By: sdesmalen, efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71252