From discussion on the mailing list: https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00615.html

The function bb_in_region contains the following check guarded by ENABLE_CHECKING:

    edge e;
    edge_iterator ei;

    /* Check that there are no edges coming in the region: all the
       predecessors of EXIT are dominated by ENTRY.  */
    FOR_EACH_EDGE (e, ei, exit->preds)
      dominated_by_p (CDI_DOMINATORS, e->src, entry);

dominated_by_p is a pure function, so the check has no effect. If it is changed to
      gcc_assert (dominated_by_p (CDI_DOMINATORS, e->src, entry));

It starts to ICE on several testcases, for example gcc.dg/graphite/block-1.c.