[llvm] [SLP] Emit reduction instead of 2 extracts + scalar op, when vectorizing operands (PR #147583)

[Philip Reames via llvm-commits]

preames wrote:

@alexey-bataev I see that you relanded this change, but don't see any description here or in the commit as to what changed.  Can you briefly describe?

https://github.com/llvm/llvm-project/pull/147583