[llvm] LAA: clarify loop-variant GEP idx computation (PR #125315)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

The title/description sound like this would just be updating comments ("clarify"), but the diff  is code changes. If this is a refactoring/cleanup, please add NFC and update the title/description. It should start with what it is doing (the "Strip getGEPInductionOperand, rework and rename stripGetElementPtr" part), then elaborate the motivation for it.

https://github.com/llvm/llvm-project/pull/125315