nir/opt_varyings: Add workaround for RADV mesh shader multiview.

The layer output is added in ac_nir_lower_ngg which is called
later than this pass; prevent deleting layer input from FS here.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28685>
6 files changed