[llvm] [IR] Speed up debug printing with persistent metadata IDs (PR #216838)

[Yaxun Liu via llvm-commits]

yxsamliu wrote:

@nikic Re: stable metadata numbering

I updated the patch so MDNodes get persistent IDs when they are created, using spare header bits. Temporary nodes get an ID when they become permanent. Function and block debug prints keep stable IDs across passes, while full module output still uses compact numbering. Compile-time-tracker is effectively flat.

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