[llvm] [LoopVectorize] Refine runtime memory check costs when there is an outer loop (PR #76034)

[David Sherwood via llvm-commits]

david-arm wrote:

> Could you update the places that look up the outer loop to use the new variable as well?

Hi @fhahn, I couldn't find any other instances in GeneratedRTChecks where we call `getParentLoop()`. I'm not sure where else I could use `OuterLoop`?

https://github.com/llvm/llvm-project/pull/76034