[VPlan] Optimize FindLast of FindIV w/o sentinel. (#172569) For FindLast reduction selecting an IV, we can avoid the horizontal AnyOf in the vector loop, by introducing an independent boolean reduction to track if the condition was ever true in the loop. If it was never true in the loop, we select the start value, otherwise the select the min/max of the FindIV reduction, as required by the predicate. The main advantage of this approach is that we have 2 independent reductions, that do not require a horizontal AnyOf reduction in the loop. Currently this requires a non-wrapping IV, but this can be relaxed in the future by selecting a canonical IV, which is then transformed to the specific derived IV for the reduction after the loop. Depends on https://github.com/llvm/llvm-project/pull/177870. PR: https://github.com/llvm/llvm-project/pull/172569
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