This loop will have maximum of three iterations. This is a lot better thanIndexOf which will search through the wholeinstructions collection which can have thousands of elements.

Should we even bother with the lookup then?
Can we measure this - if the lookup doesn't bring interesting perf wins I would vote for keeping it simple and just do the linear search over the known possible range (that should be really fast).

@vitek-karas I'm not sure about the logic and wanted to avoid potentially changing the meaning of the code, e.g, I have no idea if the dictionary lookup could yield a different result than the loop. So, I wanted to make a change that is clear it doesn't change the code semantics, and at the same time is more performant.

Regarding measuring this, I currently don't have the dotnet/runtime cloned locally to try something (the changes in this PR are done through GitHub UI actually). But I think the performance trace is quite clear this lookup is problematic, and it also the change should hopefully be making sense as a performance improvement. Yet, I understand that measuring would still be a good idea to at least know how much improvement this is. But unfortunately, I'm not able to do it right now. If necessary, would someone from the team be able to do it?