[VPlan] Remove dead recipes in entry when merging regions. (NFC)

Also remove recipes in the entry of the region that will be removed.
This makes sure we don't leave any dead users around. NFC at the moment,
but avoids causing issues in the future.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -290,6 +290,11 @@ static bool mergeReplicateRegionsIntoSuccessors(VPlan &Plan) {
       Phi1ToMove.moveBefore(*Merge2, Merge2->begin());
     }
 
+    // Remove the dead recipes in Region1's entry block.
+    for (VPRecipeBase &R :
+         make_early_inc_range(reverse(*Region1->getEntryBasicBlock())))
+      R.eraseFromParent();
+
     // Finally, remove the first region.
     for (VPBlockBase *Pred : make_early_inc_range(Region1->getPredecessors())) {
       VPBlockUtils::disconnectBlocks(Pred, Region1);