Cleanup --params for backward threader.

The new backward threader makes some of the --param knobs used to
control it questionable at best or no longer applicable at worst.

The fsm-maximum-phi-arguments param is unused and can be removed.

The max-fsm-thread-length param is block based which is a bit redundant,
since we already restrict paths based on instruction estimates.

The max-fsm-thread-paths restricts the total number of threadable paths
in a function.  We probably don't need this.  Besides, the forward
threader has no such restriction.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* doc/invoke.texi: Remove max-fsm-thread-length,
	max-fsm-thread-paths, and fsm-maximum-phi-arguments.
	* params.opt: Same.
	* tree-ssa-threadbackward.c (back_threader::back_threader): Remove
	argument.
	(back_threader_registry::back_threader_registry): Same.
	(back_threader_profitability::profitable_path_p): Remove
	param_max_fsm_thread-length.
	(back_threader_registry::register_path): Remove
	m_max_allowable_paths.

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