[llvm] [SLP]Fix/improve getSpillCost analysis (PR #129258)

[Philip Reames via llvm-commits]

preames wrote:

For additional context, this is an alternative to https://github.com/llvm/llvm-project/pull/128620 which had approached the same problem, but hit compile time concerns.  

@mikhailramalho - Would you mind running compile time measurements on this, and ensuring that it covers the original motivating case?

https://github.com/llvm/llvm-project/pull/129258