[llvm] [NFC][LoopFlatten] Lift loop versioning to function (PR #166156)

[Nashe Mncube via llvm-commits]

nasherm wrote:

@amehsan pointed out that we can achieve the same goal of this flag by setting ```loop-flatten-widen-iv=false`` and ```loop-flatten-version-loops=true```. I've verified that this works. This simplifies this patch by just making it an NFC refactor. I've updated the description and title to reflect this

https://github.com/llvm/llvm-project/pull/166156