[enhancement] opt should narrow adds where one operand has known leading/trailing zeros

### What's hard to do? (limit 100 words)

If we add two values `lhs + rhs` where `rhs` has (say) 3 trailing zeros, this can actually be simplified to `(lhs[3:] + rhs[3:]) ++ lhs[0:3]`.

XLS doesn't currently carry out these sorts of optimizations, which limits its ability to model the delay of these additions accurately, even if we would expect logic synthesis tools to spot & implement these adder optimizations.

### Current best alternative workaround (limit 100 words)

Users can manually optimize their code in these patterns, which is not ideal & risks correctness of their code

### Your view of the "best case XLS enhancement" (limit 100 words)

XLS would identify & optimize these opportunities, much as it does for addition where both operands have leading zeros.