[llvm] [LoopFlatten] Enable by default (PR #214175)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

A drive-by comment: you may want to read [the Developer Policy to enable a new pass](https://llvm.org/docs/DeveloperPolicy.html#adding-or-enabling-a-new-llvm-pass).

https://github.com/llvm/llvm-project/pull/214175