tree-optimization/111387 - BB SLP and irreducible regions

tree-optimization/111387 - BB SLP and irreducible regions

When we split an irreducible region for BB vectorization analysis
the defensive handling of external backedge defs in
vect_get_and_check_slp_defs doesn't work since that relies on
dominance info to identify a backedge.  The testcase also shows
we are iterating over the function in a sub-optimal way which is
why we split the irreducible region in the first place.  The fix
is to mark backedges and use EDGE_DFS_BACK to identify them and
to use the region RPO compute which can produce a RPO order keeping
cycles in a better order (and as side effect marks backedges).

	PR tree-optimization/111387
	* tree-vect-slp.cc (vect_get_and_check_slp_defs): Check
	EDGE_DFS_BACK when doing BB vectorization.
	(vect_slp_function): Use rev_post_order_and_mark_dfs_back_seme
	to compute RPO and mark backedges.

	* gcc.dg/torture/pr111387.c: New testcase.