[llvm] [VectorCombine] Add subvector reduction support to foldShuffleChainsToReduce (PR #199872)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

I'm trying to come up with suggestions on how simplify the code as there's a lot going on at the moment, and its not incredibly easy to grok.

https://github.com/llvm/llvm-project/pull/199872