radv: use radv_optimize_nir() less in radv_link_shaders()

fossil-db (Navi):
Totals from 11 (0.01% of 137413) affected shaders:
CodeSize: 99372 -> 99480 (+0.11%)
Instrs: 19119 -> 19110 (-0.05%)
Cycles: 222144 -> 222000 (-0.06%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <mesa/mesa!6891>