I like to read issues that have PR in order to learn how a problem could be solved.
how can I filter issues that have PRs associated with them on GitHub?
Just read the PR directly isn't a solution: you could have some PR which hasn't an issue reference.
You can search for issues which are linked to a pull request by a closing reference by using the linked:pr qualifier. The following search query is what you want:
is:issue linked:pr