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

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

I think it depends on what you are testing. Just to check that `isComputableLoopNest` is working correctly, adding a new optimization remark message seems to be enough. If you would like to check that the `DependenceInfo` is not invoked, inserting some message just before it is called, and using CHECK-NOT seems to be a way to go. (I thought using `-debug-pass-manager` would work fine, but nope).

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