[VPlan] Clean up preheader block after b85a402.

Fix a leak introduced in b85a402 and flagged by LSan
https://lab.llvm.org/buildbot#builders/5/builds/33452

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -591,6 +591,9 @@ VPlan::~VPlan() {
       Block->dropAllReferences(&DummyValue);
 
     VPBlockBase::deleteCFG(Entry);
+
+    Preheader->dropAllReferences(&DummyValue);
+    delete Preheader;
   }
   for (VPValue *VPV : VPLiveInsToFree)
     delete VPV;