nir: remove dead glsl variables before lowering io.

For cull distance GLSL will let unsized unused arrays get
into the backend, we should nuke those straight away, to
save caring about them later.

This fixes:
arb_separate_shader_objects/linker/large-number-of-unused-varyings
as a side effect (even without culling changes).

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/mesa/drivers/dri/i965/brw_nir.c

@@ -561,6 +561,7 @@ brw_create_nir(struct brw_context *brw,
    /* First, lower the GLSL IR or Mesa IR to NIR */
    if (shader_prog) {
       nir = glsl_to_nir(shader_prog, stage, options);
+      nir_remove_dead_variables(nir, nir_var_shader_in | nir_var_shader_out);
       OPT_V(nir_lower_io_to_temporaries,
             nir_shader_get_entrypoint(nir),
             true, false);