virgl, nir_to_tgsi: Add a hack for promoting partial memory barriers

virgl, nir_to_tgsi: Add a hack for promoting partial memory barriers

Most drivers will want nir_opt_barrier_modes() to optimize out
unnecessary memory barrier modes.  However, virgl has to translate
back to GLSL, which means it can really only handle partial memory
barriers in compute shaders today, because there isn't a proper
way to express them otherwise.  Just ask nir_to_tgsi to promote
these back to full barriers as a workaround.

See KHR-GL43.shader_storage_buffer_object.advanced-readWrite-case1
on virpipe-on-gl as a case where this hack is needed.

Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24842>