Do not allow irreducible loops/regions in a scop

Do not allow irreducible loops/regions in a scop

Irreducible regions are not going to be optimized by ISL
so discard them early. Passes bootstrap and regtest.

gcc/ChangeLog:

2015-11-06  Aditya Kumar  <aditya.k7@samsung.com>

        * graphite-scop-detection.c (scop_detection::merge_sese): Entry and exit edges should not be a part of irreducible loop.
        (scop_detection::can_represent_loop_1): Loops should not be irreducible.
        (scop_detection::harmful_stmt_in_region): All the basic block should belong to reducible loops.

Co-Authored-By: Sebastian Pop <s.pop@samsung.com>
From-SVN: r229888