Remove legacy back threader.

Remove legacy back threader.

At this point I don't see any use for the legacy mode, which I had
originally left in place during the transition.

This patch removes the legacy back threader, and cleans up the code a
bit.  There are no functional changes to the non-legacy code.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* doc/invoke.texi: Remove docs for threader-mode param.
	* flag-types.h (enum threader_mode): Remove.
	* params.opt: Remove threader-mode param.
	* tree-ssa-threadbackward.c (class back_threader): Remove
	path_is_unreachable_p.
	Make find_paths private.
	Add maybe_thread and thread_through_all_blocks.
	Remove reference marker for m_registry.
	Remove reference marker for m_profit.
	(back_threader::back_threader): Adjust for registry and profit not
	being references.
	(dump_path): Move down.
	(debug): Move down.
	(class thread_jumps): Remove.
	(class back_threader_registry): Remove m_all_paths.
	Remove destructor.
	(thread_jumps::thread_through_all_blocks): Move to back_threader
	class.
	(fsm_find_thread_path): Remove
	(back_threader::maybe_thread): New.
	(back_threader::thread_through_all_blocks): Move from
	thread_jumps.
	(back_threader_registry::back_threader_registry): Remove
	m_all_paths.
	(back_threader_registry::~back_threader_registry): Remove.
	(thread_jumps::find_taken_edge): Remove.
	(thread_jumps::check_subpath_and_update_thread_path): Remove.
	(thread_jumps::maybe_register_path): Remove.
	(thread_jumps::handle_phi): Remove.
	(handle_assignment_p): Remove.
	(thread_jumps::handle_assignment): Remove.
	(thread_jumps::fsm_find_control_statement_thread_paths): Remove.
	(thread_jumps::find_jump_threads_backwards): Remove.
	(thread_jumps::find_jump_threads_backwards_with_ranger): Remove.
	(try_thread_blocks): Rename find_jump_threads_backwards to
	maybe_thread.
	(pass_early_thread_jumps::execute): Same.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/ssa-dom-thread-7.c: Remove call into the legacy
	code and adjust for ranger threader.