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

[Ehsan Amiri via llvm-commits]

amehsan wrote:

> Thanks. I think doc adds more value than the code comments as it gives us an easy place to checkout the details and limitations. It also serves a quick starting point for new contributors and listing everything out clearly opens new opportunity for contributions. Reading docs is low friction that reading code from new contributors perspective.

I am fine either way.

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