Refactor backward threader registry and profitability code into classes.

Refactor backward threader registry and profitability code into classes.

This refactors the registry and the profitability code from the
backwards threader into two separate classes.  It cleans up the code,
and makes it easier for alternate implementations to share code.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c (class thread_jumps): Split out code
	from here...
	(class back_threader_registry): ...to here...
	(class back_threader_profitability): ...and here...
	(thread_jumps::thread_through_all_blocks): Remove argument.
	(back_threader_registry::back_threader_registry): New.
	(back_threader_registry::~back_threader_registry): New.
	(back_threader_registry::thread_through_all_blocks): New.
	(thread_jumps::profitable_jump_thread_path): Move from here...
	(back_threader_profitability::profitable_path_p): ...to here.
	(thread_jumps::find_taken_edge): New.
	(thread_jumps::convert_and_register_current_path): Move...
	(back_threader_registry::register_path): ...to here.
	(thread_jumps::register_jump_thread_path_if_profitable): Move...
	(thread_jumps::maybe_register_path): ...to here.
	(thread_jumps::handle_phi): Call find_taken_edge and
	maybe_register_path.
	(thread_jumps::handle_assignment): Same.
	(thread_jumps::fsm_find_control_statement_thread_paths): Remove
	tree argument to handle_phi and handle_assignment.
	(thread_jumps::find_jump_threads_backwards): Set m_name.  Remove
	set of m_speed_p and m_max_threaded_paths.
	(pass_thread_jumps::execute): Remove second argument from
	find_jump_threads_backwards.
	(pass_early_thread_jumps::execute): Same.