[llvm] [IR] Add an opt-in fast IR printing mode (PR #216838)

[Yaxun Liu via llvm-commits]

yxsamliu wrote:

@aengelke @nikic Re: stable metadata IDs

I updated the patch to follow this approach. Metadata IDs are stored in `LLVMContext` and assigned when first printed, so they stay stable across pass dumps. Final module output still uses compact numbering.

The output is lossless. This made function dumps about 2.4x faster and `-print-changed` about 7.8x faster.

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