nir: Add ability to overwrite incomplete GS primitives.

nir: Add ability to overwrite incomplete GS primitives.

After each end_primitive and at the end of the shader before emitting
set_vertex_and_primitive_count, we check if the primitive that is being
emitted has enough vertices or not, and we adjust the vertex and
primitive counters accordingly.

As a result, if the backend uses this option, the backend compiler
will not have to worry about discarding the unneeded vertices
and primitives.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6964>