Revert "fix up AVR code gen for cmp_zero"
Revert "fix up AVR code gen for cmp_zero"

This reverts commit 7d670643c595be0e6e878237a9dba9a9e8673848.

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