Extend the NOT+ADC-trick (sic) for SUB to SBB.
This should fix 403.gcc. A SBB with AM could not fulfill its should_be_same contraint, so the AM gets split from the SBB. The Load of the AM gets assigned the out register of the SBB, which violates the !in_r5 constraint of the SBB. Therefore it's necessary to turn the SBB into NOT+ADC.