[llvm] [SLP]Fix graph traversal in getSpillCost (PR #124984)

[via llvm-commits]

bgra8 wrote:

> The crash is not related to SLP vectorizer, the reproducer crashes in codegen. I suggest to create an issue again AARCH64 codegen.

Even if the crash happens somewhere else this is the patch that exposes the crash (the compiler does not crash before this change). I think the [llvm patch revert policy](https://llvm.org/docs/DeveloperPolicy.html#patch-reversion-policy) strongly encourages “reverting to green” and investigating offline to keep the tip of tree in a good state.

https://github.com/llvm/llvm-project/pull/124984