[llvm] [IR] Use persistent metadata IDs for pass snapshots (PR #216838)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

I still dislike the configuration option. In addition to the complexity this adds to the code, I don't think it's a good idea to have two different modes of printing metadata numbers -- IMO, each MDNode should have *one* number, which is the number stored inside its header, and we should only ever print *that* number. This number should replace SlotTracker's own numbering logic (and also eliminate the ShouldInitializeAllMetadata parameter).

If we want a dense numbering at the end, we can have a separate function that renumbers before printing assembly output in our tools. (What would be the impact if we don't do this? How many tests would fail?)

https://github.com/llvm/llvm-project/pull/216838