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

[Philip Reames via llvm-commits]

preames 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). 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. Namely the statement: "If a test case that demonstrates a problem is reported in the commit thread, please revert and investigate offline." should cover this.

It does.  However, this seems like a case where the investigation has been done, and the patch is unlikely to be at fault.  If *after* filing the appropriate issue, the backend bug turns out to be non-trivial, we can re-examine, but Alexey's request for you to fix a backend bug is not unreasonable here.  

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