Assert that backedges are available in path solver.

Assert that backedges are available in path solver.

gcc/ChangeLog:

	* cfganal.cc (verify_marked_backedges): New.
	* cfganal.h (verify_marked_backedges): New.
	* gimple-range-path.cc (path_range_query::path_range_query):
	Verify freshness of back edges.
	* tree-ssa-loop-ch.cc (ch_base::copy_headers): Call
	mark_dfs_back_edges.
	* tree-ssa-threadbackward.cc (back_threader::back_threader): Move
	path_range_query construction after backedges have been
	updated.