[llvm] Revert "[IR] Reduce memory usage in metadata use tracking (NFC)" (PR #228299)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> We've encountered non-determinism with this patch.

Are you sure it's non-determinism? The `dropRef` imp replaces the removed entry with the last entry. It's deterministic but it doesn't follow the original order.

https://github.com/llvm/llvm-project/pull/228299