[llvm] [IfConversion] Preserve debug info when flipping a branch (PR #114614)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I saw this comment in a few places and I figured it would be an easy improvement.
> 
> ```
> DebugLoc dl; // FIXME: this is nowhere
> ```

Should be easily MIR testable? 

https://github.com/llvm/llvm-project/pull/114614