[llvm] [VPlan] Add ReductionStartVector VPInstruction. (PR #142290)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

Came here to say the same thing: we have also bisected a correctness issue down to this change. 
It occurs in SPECv8 and 706.stockfish_r if you happen to have access to the next SPEC by any chance. Haven't tried reproducing it yet, but it looks like we already have a reproducer now.

Is this worth a revert? Or will there be a quick fix?

https://github.com/llvm/llvm-project/pull/142290