[clang] [flang] [llvm] add -floop-fuse to clang and flang (PR #142686)

[Madhur Amilkanthwar via cfe-commits]

madhur13490 wrote:

> > What do you think is the right place in pass pipeline?
> 
> I don't know, but just a gut feeling, somewhere in `buildModuleOptimizationPipeline` looks reasonable to me rather than `buildFunctionSimplificationPipeline`. However, again, it should be determined by what you'd like to achieve with loop fusion.
> 
> FWIW: recently I moved the loop interchange pass from the simplification pipeline into the optimization pipeline in #145502. I did so more due to the drawbacks of having it in the simplification pipeline than any particular advantages of placing it in the optimization pipeline. I also confirmed that the number of interchanged loops barely changed after the move. However, I found that it triggered potential phase ordering issues after merging it (in my case, some other passes like GVN/LICM can transform the IR into a form that makes interchange difficult). I believe this should be handled inside LoopInterchange itself, but it's worth noting that (I think) it's not an uncommon situation.
> 
> (That said, if you don’t have any specific motivating examples at the moment, I suppose any placement may be fine).

Yeah, I don't have enough data to support the right place in the pipeline. I will add `FIXME` in the code now. Right now, it is added before the other loop passes (like LoopInstSimplify, LoopSimplify etc.) are added in LPM. 

https://github.com/llvm/llvm-project/pull/142686