Pull Request Diff Tab - Add comment online with file :Missing descriptive labels for Add reaction button

Issue Summary

In the diff pull request section, for the line comments on a file, the "Add reaction" button has missing descriptive labels.

Steps to Reproduce

1. Launch the URL
2. In the Diff section of the code or file, navigate to the line of code or file and add a comment or go to any existing comment.
3. Navigate to the add reaction button on the comment.

Screenshot

Screen Recording

Screen Recording 2022-04-28 at 12.27.42 PM.mov

Actual Results

The buttons to "Add reaction" in the comments section for the line of code or file have a missing label. The button is announced as "Button" which does not describe the functionality attached to the button. In the absence of unique and descriptive labels, it becomes difficult for screen reader users to understand the purpose of the form control. If the purpose of the form control is not clear, users will find it difficult to interact with the control. 

Expected Results

Provide an appropriate label or title such as aria-label=" Add reaction" to the button. The screen reader should announce the label associated with the button. Label or title is helpful for users with visual disabilities, learning disabilities, users browsing with text browsers etc. to understand the information conveyed using buttons. 

Workaround

Currently, there is no known workaround for this behaviour. A workaround will be added here when available.

Bug Ref: 739294