[llvm] [LoopInterchange] Hoist isComputableLoopNest() in the control flow (PR #124247)

[Sjoerd Meijer via llvm-commits]

https://github.com/sjoerdmeijer commented:

Sorry for being a bit late, this also looks generally good to me, but had 2 questions/nits.

https://github.com/llvm/llvm-project/pull/124247