[NFC][InstCombine] Aggregate reconstruction: use plain map

Now that we no longer require for this map to have stable iteration order,
we no longer need to pay for keeping the iteration order stable,
so switch from `SmallMapVector` to `SmallDenseMap`.

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -945,7 +945,7 @@ Instruction *InstCombinerImpl::foldAggregateConstructionIntoAggregateReuse(
   // For each predecessor, what is the source aggregate,
   // from which all the elements were originally extracted from?
   // Note that we want for the map to have stable iteration order!
-  SmallMapVector<BasicBlock *, Value *, 4> SourceAggregates;
+  SmallDenseMap<BasicBlock *, Value *, 4> SourceAggregates;
   for (BasicBlock *Pred : Preds) {
     std::pair<decltype(SourceAggregates)::iterator, bool> IV =
         SourceAggregates.insert({Pred, nullptr});