[ARITH] fix zero iter bug in arith

[spectrometerHBH]

No description provided.

[tqchen]

Thanks @spectrometerHBH , is it possible to also fix via other means? e.g. can we early simplify the zero iters and ignore them in sum

[junrushao]

I agree with Tianqi, we should simplify the zeros out

[spectrometerHBH]

Actually, the bug is caused by simplifying the zero iters.

After simplifying, the left hand side of constraints (iter < bound) may only contain one IterSplit in IterSum.

We fuse the IterSum into one IterMark, and expect the fusing result to be IterSplit(IterMark(IterSum)). But if the IterSum only contain one IterSplit and we directly return the IterSplit, the result will be IterSplit(IterMark(Var)), which is not appropriate.

Also, We use a map from IterSum to IterMark to note down the correct bound induced by constraint. So we expect the source of IterMark to be an IterSum.

[junrushao]

Thanks for the explanation! The PR looks good to me now