[llvm] [SLP]Add GEP cost estimation for gathered loads. (PR #80867)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

I'm approving because the code change makes sense, the test delta on the example is actually a regression and you probably want to follow up on why.

https://github.com/llvm/llvm-project/pull/80867