nir: initialize uses_discard to false

This matches what we do for uses_sample_qualifier, and what we
do in ir_set_program_inouts.cpp as well.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir_gather_info.c

@@ -381,6 +381,7 @@ nir_shader_gather_info(nir_shader *shader, nir_function_impl *entrypoint)
    }
    if (shader->info.stage == MESA_SHADER_FRAGMENT) {
       shader->info.fs.uses_sample_qualifier = false;
+      shader->info.fs.uses_discard = false;
    }
 
    void *dead_ctx = ralloc_context(NULL);