intel/compiler: Combine nir_emit_{ssbo,shared}_atomic into one helper

intel/compiler: Combine nir_emit_{ssbo,shared}_atomic into one helper

These are basically identical save for:
- shared has surface hardcoded to SLM rather than an SSBO index
- shared has to handle adding the 'base' const_index (SSBO have none)
- the NIR source index for data is shifted by one

It's not worth copy and pasting the entire function for this.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rohan Garg <rohan.garg@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20604>