The property you have here is that the add between base.off and idx is nusw. In general, this implies neither nuw, nor nsw. If you know that idx is non-negative (or the original GEP is nuw) then you can infer nuw.
Some examples to illustrate:
- Consider base.off=0x7fffffff, idx=1. Adding these is nuw, but not nsw.
- Consider base.off=1, idx=-1. Adding these is nsw, but not nuw.
Both are nusw (i.e., no wrap if we interpret the LHS as unsigned and RHS as signed).