Improve splitX passes management

Improve splitX passes management

The names of split_before_sched2 ("split4") and split_before_regstack
("split3") do not reflect their insertion point in the sequence of passes,
where split_before_regstack follows split_before_sched2. Reorder the code
and rename the passes to reflect the reality.

split_before_regstack pass does not need to run if split_before_sched2 pass
was already performed. Introduce enable_split_before_sched2 function to
simplify gating functions of these two passes.

There is no need for a separate rest_of_handle_split_before_sched2.
split_all_insns can be called unconditionally from
pass_split_before_sched2::execute, since the corresponding gating function
determines if the pass is executed or not.

	* recog.c: Move pass_split_before_sched2 code in front of
	pass_split_before_regstack.
	(pass_data_split_before_sched2): Rename pass to split3 from split4.
	(pass_data_split_before_regstack): Rename pass to split4 from split3.
	(rest_of_handle_split_before_sched2): Remove.
	(pass_split_before_sched2::execute): Unconditionally call
	split_all_insns.
	(enable_split_before_sched2): New function.
	(pass_split_before_sched2::gate): Use enable_split_before_sched2.
	(pass_split_before_regstack::gate): Ditto.
	* config/nds32/nds32.c (nds32_split_double_word_load_store_p):
	Update name check for renamed split4 pass.
	* config/sh/sh.c (register_sh_passes): Update pass insertion
	point for renamed split4 pass.