radv: move nir_opt_conditional_discard out of optimization loop

radv: move nir_opt_conditional_discard out of optimization loop

This late optimization pass is only affected by nir_opt_if() and handles all cases
in a single pass. It's enough to call it once after the optimization loop.
No changes on vkpipeline-db.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>