[llvm] [SimplfyCFG] Set `MD_prof` for `select` used for certain conditional simplifications (PR #154426)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> I'm not sure on this one. The thing is that the canonical form here is a plain and/or, and the only reason we use select is to prevent poison-propagation issues. Improvements to frontend information (e.g. if you slap noundef on the relevant arguments) or analysis may end up converting the select to and/or, in which case we won't be able to preserve the prof metadata. Cases where better analysis has adverse effects are always awkward.
> Do you have an example where duplicating this information on the select ends up being useful? After all, these weights are already on the branch over the select.

The approach is defensive rather than example-driven, because at large scale, places where profile information starts deteriorating are hard to identify, and it's also hard to know there's a problem - hence the approach of "if an instruction can have MD_profile, and it's known, let's capture it, otherwise mark it as `unknown`". This is compounded by changes that break downstream assumptions. In this case, the profile info is easily known, so it seems prudent to capture it rather than marking it unknown.

Not sure what you mean by "better analysis has adverse effects"?

https://github.com/llvm/llvm-project/pull/154426