[LV] Use LatchVPBB directly instead of going through region (NFC).

Split off from D158333.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8918,8 +8918,7 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   // ---------------------------------------------------------------------------
 
   // Adjust the recipes for any inloop reductions.
-  adjustRecipesForReductions(cast<VPBasicBlock>(TopRegion->getExiting()), Plan,
-                             RecipeBuilder, Range.Start);
+  adjustRecipesForReductions(LatchVPBB, Plan, RecipeBuilder, Range.Start);
 
   // Interleave memory: for each Interleave Group we marked earlier as relevant
   // for this VPlan, replace the Recipes widening its memory instructions with a