[llvm] [IR] Reduce memory usage in metadata use tracking (NFC) (PR #221866)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

I'll have a closer look in a few days, I'm a bit short on time these days.

(Also, I believe we have a weekly pinging policy.)

https://github.com/llvm/llvm-project/pull/221866