[llvm] [LoopRotate] Rotate loop if this makes exit count computable (PR #162654)

[Marek Sedláček via llvm-commits]

mark-sed wrote:

> Don't know whether that's due to the transform itself or whether it's because this enables expensive followup transforms.

@nikic That was unexpected. Do you have any recommendation on what to do about this?
We can see that this brings potential speedup with unrolling and vectorization where it was not possible and I am not sure what is justifiable trade off of compile time for performance.

One option I could propose is enabling this only for higher level opt.
Alternatively it could be controlled by off-by-default flag.

https://github.com/llvm/llvm-project/pull/162654