[llvm] [Github][RFC] Add workflow to diff codegen on llvm-test-suite (PR #190010)

[Luke Lau via llvm-commits]

lukel97 wrote:

@kwk thanks for the review, having a persistent comment that gets updated with the progress does sound like a good idea.

But similar to reactions I think that would also require the `issue: write` permission. I believe it's possible to update an existing comment with the `issue-write.yml` workflow, but I think we need to kick off multiple workflows in between steps. Since `issue-write.yml` only gets triggered when a workflow finishes. I'll try and look into this in a follow up PR. 

> 2\. If the PR is not mergeable you can append to the comment:

FWIW this PR should already post a comment if the PR isn't mergeable

https://github.com/llvm/llvm-project/pull/190010