nir: intel/compiler: Add and use nir_op_pack_32_4x8_split

nir: intel/compiler: Add and use nir_op_pack_32_4x8_split

A lot of CTS tests write a u8vec4 or an i8vec4 to an SSBO.  This results
in a lot of shifts and MOVs.  When that pattern can be recognized, the
individual 8-bit components can be packed much more efficiently.

v2: Rebase on b4369de27fc ("nir/lower_packing: use
shader_instructions_pass")

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9025>