intel/fs: Combine adjacent memory barriers

intel/fs: Combine adjacent memory barriers

This will avoid generating multiple identical fences in a row.

For Gen11+ we have multiple types of fences (affecting different
variable modes), but is still better to combine them in a single
scoped barrier so that the translation to backend IR have the option
of dispatching both fences in parallel.

This will clean up redundant barriers from various
dEQP-VK.memory_model.* tests.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3224>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3224>