in build_sese_loop_nests, the loops in sese->loop_nest can be:
loop3 loop2 loop1(test case: gcc.dg/graphite/block-3.c), after
the algorithms(sort the loops) there,
it will still unordered as loop2 loop1 loop3.
I'm not sure if this is the bug. but if this bug is confirmed,
I will take care of this.