[llvm] [Reassociate][DebugInfo] Salvage debug info before rewriting expression (PR #182730)

[J. Ryan Stinnett via llvm-commits]

https://github.com/jryans requested changes to this pull request.

Looks like the core of the change is correct, thanks for working on this! 😄 

I think it would be good to also add a new test from one of the fixed issues (probably in a reduced form) to ensure they are covered.

---

> And now can we remove this safely? this seems like a dead code as modified instructions has already been salvaged before their operands were rewritten.
> ```
> if (ClearFlags)
>         replaceDbgUsesWithUndef(ExpressionChangedStart);
> ```

Yes, I think this can now be removed.

https://github.com/llvm/llvm-project/pull/182730