[llvm] [LoopInterchange] Fix overflow in cost calculation (PR #111807)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

Have addressed the comments about the test case.
As mentioned, I haven't renamed the type, but if you think it's better I am happy to rename things.

https://github.com/llvm/llvm-project/pull/111807