nir: add fall through comment to nir_gather_info

This stops Coverity reporting a defect and helps make the code less
error-prone.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_gather_info.c

@@ -288,6 +288,7 @@ gather_intrinsic_info(nir_intrinsic_instr *instr, nir_shader *shader,
    case nir_intrinsic_end_primitive_with_counter:
       assert(shader->info.stage == MESA_SHADER_GEOMETRY);
       shader->info.gs.uses_end_primitive = 1;
+      /* fall through */
 
    case nir_intrinsic_emit_vertex:
       if (nir_intrinsic_stream_id(instr) > 0)