[identity_op]: Fix duplicate diagnostics

[cookie-s]

Relates to #12379

In the identity_op lint, the following diagnostic was emitted two times

  --> tests/ui/identity_op.rs:156:5
   |
LL |     1 * 1;
   |     ^^^^^ help: consider reducing it to: `1`
   |

because both of the left operand and the right operand are the identity element of the multiplication.

This PR fixes the issue so that if a diagnostic is created for an operand, the check of the other operand will be skipped. It's fine because the result is always the same in the affected operators.

---

changelog: [identity_op]: Fix duplicate diagnostics

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Alexendoo (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[Alexendoo]

Thanks! @bors r+

[bors]

📌 Commit 3b9939e has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3b9939e with merge c0939b1...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing c0939b1 to master...