Fix computation of single entry/exit of a region.

Fix computation of single entry/exit of a region.

For basic block with two preds, allow (as single entry) only when the other
edge is a backedge. Similarly for basic block with two succs,
allow (as single exit) only when the other edge is a back edge.

2015-12-21  Aditya Kumar  <hiraditya@msn.com>

        * graphite-scop-detection.c (scop_detection::get_nearest_dom_with_single_entry): Check l == l2.
        (scop_detection::get_nearest_pdom_with_single_exit): Same.
        (scop_detection::merge_sese): Whitespace.
        (scop_detection::add_scop): Comment.
        (build_scops): Whitespace.

From-SVN: r231946