aco/gfx11: fix discard early exit removal optimization
This optimization never happened because the NULL target was removed in GFX11.
fossil-db (gfx1100):
Totals from 5547 (4.12% of 134574) affected shaders:
Instrs: 418304 -> 397198 (-5.05%)
CodeSize: 2218664 -> 2114216 (-4.71%)
Latency: 3499056 -> 3392977 (-3.03%)
InvThroughput: 275941 -> 268645 (-2.64%)
Branches: 8682 -> 3135 (-63.89%)