Move statics to threader pass class.

Move statics to threader pass class.

This patch moves all the static functions into the pass class, and
cleans up things a little.  The goal is to shuffle things around such
that we can add debug counters that depend on different threading
passes, but it's a clean-up on its own right.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c (BT_NONE): New.
	(BT_SPEED): New.
	(BT_RESOLVE): New.
	(back_threader::back_threader): Add flags.
	Move loop initialization here.
	(back_threader::~back_threader): New.
	(back_threader::find_taken_edge_switch): Change solver and ranger
	to pointers.
	(back_threader::find_taken_edge_cond): Same.
	(back_threader::find_paths_to_names): Same.
	(back_threader::find_paths): Same.
	(back_threader::dump): Same.
	(try_thread_blocks): Merge into thread_blocks.
	(back_threader::thread_blocks): New.
	(do_early_thread_jumps): Merge into thread_blocks.
	(do_thread_jumps): Merge into thread_blocks.
	(back_threader::thread_through_all_blocks): Remove.