Add debug (slp_tree) and dump infrastructure for this

Add debug (slp_tree) and dump infrastructure for this

This adds an alternate debug_dump_context similar to the one for
selftests but for interactive debugging routines.  This allows
to share code between user-visible dumping via the dump_* API
and those debugging routines.  The primary driver was SLP node
dumping which wasn't accessible from inside a gdb session up to
now.

2020-05-27  Richard Biener  <rguenther@suse.de>

	* dump-context.h (debug_dump_context): New class.
	(dump_context): Make it friend.
	* dumpfile.c (debug_dump_context::debug_dump_context):
	Implement.
	(debug_dump_context::~debug_dump_context): Likewise.
	* tree-vect-slp.c: Include dump-context.h.
	(vect_print_slp_tree): Dump a single SLP node.
	(debug): New overload for slp_tree.
	(vect_print_slp_graph): Rename from vect_print_slp_tree and
	use that.
	(vect_analyze_slp_instance): Adjust.