cse: Change return type of predicate functions from int to bool

cse: Change return type of predicate functions from int to bool

Also change some function arguments to bool and remove one instance
of always zero function argument.

gcc/ChangeLog:

	* rtl.h (exp_equiv_p): Change return type from int to bool.
	* cse.cc (mention_regs): Change return type from int to bool
	and adjust function body accordingly.
	(exp_equiv_p): Ditto.
	(insert_regs): Ditto. Change "modified" function argument to bool
	and update usage accordingly.
	(record_jump_cond): Remove always zero "reversed_nonequality"
	function argument and update usage accordingly.
	(fold_rtx): Change "changed" variable to bool.
	(record_jump_equiv): Remove unneeded "reversed_nonequality" variable.
	(is_dead_reg): Change return type from int to bool.