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

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

Also change some internal variables from int to bool.

gcc/ChangeLog:

	* cprop.cc (reg_available_p): Change return type from int to bool.
	(reg_not_set_p): Ditto.
	(try_replace_reg): Ditto.  Change "success" variable to bool.
	(cprop_jump): Change return type from int to void
	and adjust function body accordingly.
	(constprop_register): Ditto.
	(cprop_insn): Ditto.  Change "changed" variable to bool.
	(local_cprop_pass): Change return type from int to void
	and adjust function body accordingly.
	(bypass_block): Ditto.  Change "change", "may_be_loop_header"
	and "removed_p" variables to bool.
	(bypass_conditional_jumps): Change return type from int to void
	and adjust function body accordingly.  Change "changed"
	variable to bool.
	(one_cprop_pass): Ditto.