[Forwardport] Refactor: remove some code duplication

[arnoudhgz]

Original Pull Request

#17505

• Complex statement that was executed three times moved to separate
  method so the logic is only on one place
• The calculation for the index of the last record moved to a variable
  so the calculation is only being done once. This also reduces the amount
  of times this.getChildItems() is being called.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @arnoudhgz. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

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[magento-engcom-team]

Hi @arnoudhgz. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.