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

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

> Would it make sense to use `InstructionCost` which already accounts for saturation?

Thanks for the suggestion, I think that would make a lot of sense, which might as simple as aliasing CacheCostTy to InstructionCost (it's just an int at the moment: `using CacheCostTy = int64_t;`). Will look into it.  


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