intel/compiler: adjust task payload offsets as late as possible

intel/compiler: adjust task payload offsets as late as possible

Otherwise passes which expect offsets to be in bytes (like
brw_nir_lower_mem_access_bit_sizes, called from brw_postprocess_nir)
may produce incorrect results.

Fixes 64-bit load/stores in task/mesh shaders.

Fixes: c36ae42e4cc ("intel/compiler: Use nir_var_mem_task_payload")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16196>