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GH-34529: [C++][Compute] Replace explicit checking with DCHECK for invariants in row segmenter (#44236)



### Rationale for this change

As #34529 described.

### What changes are included in this PR?

Change the `if` checking for invariants in row segmenter to `DCHECK`.

### Are these changes tested?

No need.

### Are there any user-facing changes?

None.

* GitHub Issue: #34529

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>