[llvm] [LoopFusion] Document LoopFusion Pass (PR #192926)

[Madhur Amilkanthwar via llvm-commits]

madhur13490 wrote:

> there is also this CI failure:
> 
> ```
> Warning, treated as error:
> /home/runner/work/llvm-project/llvm-project/llvm/docs/LoopFusion.rst:document isn't included in any toctree
> ninja: build stopped: subcommand failed.
> Error: Process completed with exit code 2.
> ```

I will take care of this while merging.

https://github.com/llvm/llvm-project/pull/192926