graphite: Extend SCoP detection dump output

graphite: Extend SCoP detection dump output

Extend dump output to make understanding why Graphite rejects to
include a loop in a SCoP easier (for GCC developers).

gcc/ChangeLog:

        * graphite-scop-detection.cc (scop_detection::can_represent_loop):
	Output reason for failure to dump file.
        (scop_detection::harmful_loop_in_region): Likewise.
        (scop_detection::graphite_can_represent_expr): Likewise.
        (scop_detection::stmt_has_simple_data_refs_p): Likewise.
        (scop_detection::stmt_simple_for_scop_p): Likewise.
	(print_sese_loop_numbers): New function.
        (scop_detection::add_scop): Use from here to print loops in
	rejected SCoP.