emit-rtl: Change return type of predicate functions from int to bool

Also fix some stalled comments.

gcc/ChangeLog:

	* rtl.h (subreg_lowpart_p): Change return type from int to bool.
	(active_insn_p): Ditto.
	(in_sequence_p): Ditto.
	(unshare_all_rtl): Change return type from int to void.
	* emit-rtl.h (mem_expr_equal_p): Change return type from int to bool.
	* emit-rtl.cc (subreg_lowpart_p): Change return type from int to bool
	and adjust function body accordingly.
	(mem_expr_equal_p): Ditto.
	(unshare_all_rtl): Change return type from int to void
	and adjust function body accordingly.
	(verify_rtx_sharing): Remove unneeded return.
	(active_insn_p): Change return type from int to bool
	and adjust function body accordingly.
	(in_sequence_p): Ditto.