fwprop: invoke change_is_worthwhile to judge if a replacement is worthwhile

fwprop: invoke change_is_worthwhile to judge if a replacement is worthwhile

gcc/
	* fwprop.cc (try_fwprop_subst_pattern): Invoke change_is_worthwhile
	to judge if a replacement is worthwhile.  Remove single_set check
	and add is_debug_insn check.
	* recog.cc (swap_change): Invalidate recog_data when the cached INSN
	is swapped out.
	* rtl-ssa/changes.cc (rtl_ssa::changes_are_worthwhile): Check if the
	insn cost of new rtl is unknown and fail the replacement.