This is split from D55452
Pairwise reductions require two shuffles on every level but the last. On the last level the two shuffles are <1, u, u, u...> and <0, u, u, u...>, but <0, u, u, u...> will be dropped by InstCombine/DAGCombine as being an identity shuffle.
Is this really required for this patch? I think not. This patch should do only the declared thing - fix of the cost, nothing else. This change is not required for the fix of the cost model.