[LV] Account for vp_merge in out of loop EVL reductions in legacy cost model (#115903) In #101641, support for out of loop reductions with EVL tail folding was added by transforming selects to vp_merges in transformRecipestoEVLRecipes. Whilst the select was previously free, the vp_merge wasn't and incurs a cost on RISC-V with the VPlan cost model. But this diverged from the legacy cost model and caused the "VPlan cost model and legacy cost model disagreed" assertion to trigger when building 502.gcc_r from SPEC CPU 2017. Neither the select nor vp_merge recipes from the VPlan exist in the underlying instructions, so I thought it would make the most sense to fix this by adding the cost to the underlying phi instruction in getInstructionCost. It's worth noting that on RISC-V this vp_merge won't actually generate any instructions because the mask is all true, and will be folded away. So we should update the cost model at some point to reflect that.
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