[LV] Propagate PredicatedBBsAfterVectorization to predecessors.

This fixes some cases where predicated BBs where missed previously,
leading to under-estimating the cost of those blocks.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -5824,6 +5824,10 @@ void LoopVectorizationCostModel::collectInstsToScalarize(ElementCount VF) {
           ScalarCostsVF.insert(ScalarCosts.begin(), ScalarCosts.end());
         // Remember that BB will remain after vectorization.
         PredicatedBBsAfterVectorization[VF].insert(BB);
+        for (auto *Pred : predecessors(BB)) {
+          if (Pred->getSingleSuccessor() == BB)
+            PredicatedBBsAfterVectorization[VF].insert(Pred);
+        }
       }
   }
 }