fix up AVR code gen for single instructions (#766)
fix up AVR code gen for single instructions (#766)


switch-education/pxt,switchinnovations/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,switch-education/pxt,playi/pxt,switchinnovations/pxt,playi/pxt,switch-education/pxt,playi/pxt,Microsoft/pxt,playi/pxt,switchinnovations/pxt,Microsoft/pxt,Microsoft/pxt,Microsoft/pxt,switch-education/pxt