nir/lower_blend: Optimize masked out RTs

While debugging KHR-GLES31.core.draw_buffers_indexed.color_masks, the noise from piles of store_output(load_output) instructions got in the way. Optimize it out.

This does not fix the test, but if this case ever happened in a real app it would improve performance. This is only load bearing on Asahi (and PanVK?), since Panfrost wouldn't call nir_lower_blend at all in this case.