[llvm] [VPlan] Introduce ComputeReductionResult VPInstruction opcode. (PR #70253)

[Florian Hahn via llvm-commits]

fhahn wrote:

Note that the order of the reduction result computation changed in some tests, as now we create them in the original order the reduction recipes have been created

https://github.com/llvm/llvm-project/pull/70253