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

[Mikhail R. Gadelha via llvm-commits]

mikhailramalho wrote:

> For additional context, this is an alternative to #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?

Yes, I'll run the compile time measure right now, and start a full run of SPEC.

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