Post comment on llvm:instcombine + missed-optimization issues

I propose posting an automated comment on issues tagged `llvm:instcombine` and `missed-optimization`, with contents along these lines:

> This issue has been tagged as a missed optimization in InstCombine.
>
> For the issue submitter: Please ensure that the issue explains in which real-world context the optimization is useful. Link to specific code if possible. If there is no real-world motivation, explain how you found this missed optimization (e.g. via fuzzing, looking at optimizations in other compilers, etc.) and why you think the optimization is worth implementing.
>
> For contributors:
> * Check that the issue has a real-world motivation. If it does *not* have one, do *not* create a PR without prior explicit approval from InstCombine maintainers.
> * Follow the [InstCombine contributor guide](http://llvm.org/docs/InstCombineContributorGuide.html).

This is to stop the bleeding of "well-meaning but misguided person creates a missed optimization issue based on an invented optimization opportunity" -> "someone opens a vibeslop PR against InstCombine to fix it" -> "PR gets ignored".

@dtcxzyw does this sound reasonable?

Start by reviewing the llvm:instcombine and missed-optimization issue labels and the linked InstCombine contributor guide. Identify the repository-supported entry point for automated issue comments, then verify that the proposed message is posted only for both labels and includes the requested guidance.

Written by the indexing model from the issue text.