[LoopPred] Robustly handle partially unswitched loops

We may end up with a case where we have a widenable branch above the loop, but not all widenable branches within the loop have been removed.  Since a widenable branch inhibit SCEVs ability to reason about exit counts (by design), we have a tradeoff between effectiveness of this optimization and allowing future widening of the branches within the loop.  LoopPred is thought to be one of the most important optimizations for range check elimination, so let's pay the cost.