rs6000: Don't use REGNO in the add<mode>3 expander

rs6000: Don't use REGNO in the add<mode>3 expander

The argument could be a subreg of reg instead, which means we have to
use reg_or_subregno instead of REGNO.

	* config/rs6000/rs6000.md (add<mode>3): Use reg_or_subregno instead
	of REGNO.

From-SVN: r249213