nir: Add a way to identify per-primitive variables

nir: Add a way to identify per-primitive variables

Per-primitive is similar to per-vertex attributes, but applies to all
fragments of the primitive without any interpolation involved.

Because they are regular input and outputs, keep track in shader_info
of which I/O is per-primitive so we can distinguish them after deref
lowering.  These fields can be used combined with the regular
`inputs_read`, `outputs_written` and `outputs_read`.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10600>