Make graph dumping work for fn != cfun

Make graph dumping work for fn != cfun

The following makes dumping of a function as graph work as intended
when specifying a function other than cfun.  Unfortunately the loop
and the dominance APIs are not set up to work for other functions
than cfun so you won't get any fancy loop dumps but the non-loop
dump works up to reaching mark_dfs_back_edges which I trivially made
function aware and adjusted current callers with a wrapper.

With all this, doing dot-fn id->src_cfun from the debugger when
debugging inlining works.  Previously you got a strange mix of
the src and dest functions visualized ;)

2022-01-28  Richard Biener  <rguenther@suse.de>

	* cfganal.h (mark_dfs_back_edges): Provide API with struct
	function argument.
	* cfganal.cc (mark_dfs_back_edges): Take a struct function
	to work on, add a wrapper passing cfun.
	* graph.cc (draw_cfg_nodes_no_loops): Replace stray cfun
	uses with fun which is already passed.
	(draw_cfg_edges): Likewise.
	(draw_cfg_nodes_for_loop): Do not use draw_cfg_nodes_for_loop
	for fun != cfun.