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

[Philip Reames via llvm-commits]

https://github.com/preames commented:

This generally looks fine, but one of the test changes shows a problematic interaction.  I think we need to fix the other part, then rebase this.  

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