[llvm] [MergeFunctions] Preserve instruction-level profile metadata during merging (PR #208009)

[Alok Kumar Sharma via llvm-commits]

alokkrsharma wrote:

> What's the motivation for this PR?

The motivation is mostly that MergeFunctions is a useful size optimization and we'd like it to coexist cleanly with PGO. While looking through the pass, I noticed that instruction-level !prof metadata gets dropped during merging. Since later optimizations may rely on that information, preserving it seems like the more sensible behavior. This is mainly a cleanup/completeness improvement.

https://github.com/llvm/llvm-project/pull/208009