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

[Luke Lau via llvm-commits]

lukel97 wrote:

I can confirm we're seeing a 9% improvement on 544.nab_r after this, as well as an improvement on xz_r and povray_r: https://lnt.lukelau.me/db_default/v4/nts/288

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