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

Also change some internal variables and function arguments from int to bool.

gcc/ChangeLog:

	* combine.cc (struct reg_stat_type): Change last_set_invalid to bool.
	(cant_combine_insn_p): Change return type from int to bool and adjust
	function body accordingly.
	(can_combine_p): Ditto.
	(combinable_i3pat): Ditto.  Change "i1_not_in_src" and "i0_not_in_src"
	function arguments from int to bool.
	(contains_muldiv): Change return type from int to bool and adjust
	function body accordingly.
	(try_combine): Ditto. Change "new_direct_jump" pointer function
	argument from int to bool.  Change "substed_i2", "substed_i1",
	"substed_i0", "added_sets_0", "added_sets_1", "added_sets_2",
	"i2dest_in_i2src", "i1dest_in_i1src", "i2dest_in_i1src",
	"i0dest_in_i0src", "i1dest_in_i0src", "i2dest_in_i0src",
	"i2dest_killed", "i1dest_killed", "i0dest_killed", "i1_feeds_i2_n",
	"i0_feeds_i2_n", "i0_feeds_i1_n", "i3_subst_into_i2", "have_mult",
	"swap_i2i3", "split_i2i3" and "changed_i3_dest" variables
	from int to bool.
	(subst): Change "in_dest", "in_cond" and "unique_copy" function
	arguments from int to bool.
	(combine_simplify_rtx): Change "in_dest" and "in_cond" function
	arguments from int to bool.
	(make_extraction): Change "unsignedp", "in_dest" and "in_compare"
	function argument from int to bool.
	(force_int_to_mode): Change "just_select" function argument
	from int to bool.  Change "next_select" variable to bool.
	(rtx_equal_for_field_assignment_p): Change return type from
	int to bool and adjust function body accordingly.
	(merge_outer_ops): Ditto.  Change "pcomp_p" pointer function
	argument from int to bool.
	(get_last_value_validate): Change return type from int to bool
	and adjust function body accordingly.
	(reg_dead_at_p): Ditto.
	(reg_bitfield_target_p): Ditto.
	(combine_instructions): Ditto.  Change "new_direct_jump"
	variable to bool.
	(can_combine_p): Change return type from int to bool
	and adjust function body accordingly.
	(likely_spilled_retval_p): Ditto.
	(can_change_dest_mode): Change "added_sets" function argument
	from int to bool.
	(find_split_point): Change "unsignedp" variable to bool.
	(simplify_if_then_else): Change "comparison_p" and "swapped"
	variables to bool.
	(simplify_set): Change "other_changed" variable to bool.
	(expand_compound_operation): Change "unsignedp" variable to bool.
	(force_to_mode): Change "just_select" function argument
	from int to bool.  Change "next_select" variable to bool.
	(extended_count): Change "unsignedp" function argument to bool.
	(simplify_shift_const_1): Change "complement_p" variable to bool.
	(simplify_comparison): Change "changed" variable to bool.
	(rest_of_handle_combine): Change return type to void.