Refactor back_threader_profitability

The following refactors profitable_path_p in the backward threader,
splitting out parts that can be computed once the exit block is known,
parts that contiguously update and that can be checked allowing
for the path to be later identified as FSM with larger limits,
possibly_profitable_path_p, and final checks done when the whole
path is known, profitable_path_p.

I've removed the back_threader_profitability instance from the
back_threader class and instead instantiate it once per path
discovery.  I've kept the size compute non-incremental to simplify
the patch and not worry about unwinding.

There's key changes to previous behavior - namely we apply
the param_max_jump_thread_duplication_stmts early only when
we know the path cannot become an FSM one (multiway + thread through
latch) but make sure to elide the path query when we we didn't
yet discover that but are over this limit.  Similarly the
speed limit is now used even when we did not yet discover a
hot BB on the path.  Basically the idea is to only stop path
discovery when we know the path will never become profitable
but avoid the expensive path range query when we know it's
currently not.

I've done a few cleanups, merging functions, on the way.

	* tree-ssa-threadbackward.cc
	(back_threader_profitability): Split profitable_path_p
	into possibly_profitable_path_p and itself, keep state
	as new members.
	(back_threader::m_profit): Remove.
	(back_threader::find_paths): Likewise.
	(back_threader::maybe_register_path): Take profitability
	instance as parameter.
	(back_threader::find_paths_to_names): Likewise.  Use
	possibly_profitable_path_p and avoid the path range query
	when the path is currently too large.
	(back_threader::find_paths): Fold into ...
	(back_threader::maybe_thread_block): ... this.
	(get_gimple_control_stmt): Remove.
	(back_threader_profitability::possibly_profitable_path_p):
	Split out from profitable_path_p, do early profitability
	checks.
	(back_threader_profitability::profitable_path_p): Do final
	profitability path after the taken edge has been determined.

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