[llvm] [SLP]Improved reduction cost/codegen (PR #118293)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> Could you rebase this on ToT? I tried to patch this in and resolve conflicts and I'm seeing a crash on https://github.com/google/highway/blob/master/hwy/contrib/sort/vqsort_i16a.cc. I might have missed something during conflict resolution, any chance you could test that?

Tried several different flags, cannot reproduce a crash, need a reproducer

https://github.com/llvm/llvm-project/pull/118293