[LoopDeletion] Add back statistic update lost in 523573e

Caught by a couple of builders as an unused variable warning (e.g. https://lab.llvm.org/buildbot#builders/57/builds/13973).

Author: preames

llvm/lib/Transforms/Scalar/LoopDeletion.cpp

@@ -417,6 +417,7 @@ breakBackedgeIfNotTaken(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
         return LoopDeletionResult::Unmodified;
     }
   }
+  ++NumBackedgesBroken;
   breakLoopBackedge(L, DT, SE, LI, MSSA);
   return LoopDeletionResult::Deleted;
 }