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.