[llvm] [VPlan] Add the cost of spills when considering register pressure (PR #179646)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Thanks for working on this, I remember this being discussed at the time the VPlan register pressure stuff initially landed. The load+store per register over heuristic seems sensible to me

https://github.com/llvm/llvm-project/pull/179646