nir: Prepare nir_lower_io_vars_to_temporaries() for optional PLS lowering

Rather than adding another boolean to optionally lower PLS vars, pass
the types we want to lowers through a nir_variable_mode bitmask.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Eric R. Smith <eric.smith@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37110>

src/broadcom/vulkan/v3dv_pipeline.c

@@ -315,7 +315,7 @@ preprocess_nir(nir_shader *nir)
    }
 
    NIR_PASS(_, nir, nir_lower_io_vars_to_temporaries,
-            nir_shader_get_entrypoint(nir), true, false);
+            nir_shader_get_entrypoint(nir), nir_var_shader_out);
 
    NIR_PASS(_, nir, nir_lower_system_values);