dded assertion if ldstopt() is called with nodes floating, we need precise block info

[r6148]

ir/opt/ldstopt.c

@@ -869,6 +869,8 @@ void optimize_load_store(ir_graph *irg)
   walk_env_t env;
 
   assert(get_irg_phase_state(irg) != phase_building);
+  assert(get_irg_pinned(irg) != op_pin_state_floats &&
+    "LoadStore optimization needs pinned graph");
 
   if (!get_opt_redundant_LoadStore())
     return;