[NFCI][DomTreeUpdater] applyUpdates(): reserve space for updates first

While, indeed, we may end up pushing less updates that we'd reserve space
for, self-dominating updates aren't often enough for that to matter.
But this should matter for normal updates.

lib/Analysis/DomTreeUpdater.cpp

@@ -232,6 +232,7 @@ void DomTreeUpdater::applyUpdates(ArrayRef<DominatorTree::UpdateType> Updates) {
     return;
 
   if (Strategy == UpdateStrategy::Lazy) {
+    PendUpdates.reserve(PendUpdates.size() + Updates.size());
     for (const auto &U : Updates)
       if (!isSelfDominance(U))
         PendUpdates.push_back(U);