[DomTree] Use SmallVector<DomTreeNodeBase *, 4> instead of std::vector.

Currentl DomTreeNodeBase is using std::vectot to store it's children.
Using SmallVector should be more efficient in terms of compile-time.

A size of 4 seems to be the sweet-spot in terms of compile-time,
according to

http://llvm-compile-time-tracker.com/compare.php?from=9933188c90615c9c264ebb69117f09726e909a25&to=d7a801d027648877b20f0e00e822a7a64c58d976&stat=instructions

This results in the following geomean improvements

```
                       geomean insts     max rss
O3                          -0.31 %       +0.02 %
ReleaseThinLTO              -0.35 %       -0.12 %
ReleaseLTO                  -0.28 %       -0.12 %
O0                          -0.06 %       -0.02 %
NewPM O3                    -0.36 %       +0.05 %
ReleaseThinLTO (link only)  -0.44 %       -0.10 %
ReleaseLTO-g (link only):   -0.32 %       -0.03 %
```

I am not sure if there's any other benefits of using std::vector over
SmallVector.

Reviewed By: kuhar, asbirlea

Differential Revision: https://reviews.llvm.org/D87319

llvm/include/llvm/Support/GenericDomTree.h

@@ -38,7 +38,6 @@
 #include <memory>
 #include <type_traits>
 #include <utility>
-#include <vector>
 
 namespace llvm {
 
@@ -61,17 +60,17 @@ template <class NodeT> class DomTreeNodeBase {
   NodeT *TheBB;
   DomTreeNodeBase *IDom;
   unsigned Level;
-  std::vector<DomTreeNodeBase *> Children;
+  SmallVector<DomTreeNodeBase *, 4> Children;
   mutable unsigned DFSNumIn = ~0;
   mutable unsigned DFSNumOut = ~0;
 
  public:
   DomTreeNodeBase(NodeT *BB, DomTreeNodeBase *iDom)
       : TheBB(BB), IDom(iDom), Level(IDom ? IDom->Level + 1 : 0) {}
 
-  using iterator = typename std::vector<DomTreeNodeBase *>::iterator;
+  using iterator = typename SmallVector<DomTreeNodeBase *, 4>::iterator;
   using const_iterator =
-      typename std::vector<DomTreeNodeBase *>::const_iterator;
+      typename SmallVector<DomTreeNodeBase *, 4>::const_iterator;
 
   iterator begin() { return Children.begin(); }
   iterator end() { return Children.end(); }
@@ -837,7 +836,7 @@ class DominatorTreeBase {
            "NewBB should have a single successor!");
     NodeRef NewBBSucc = *GraphT::child_begin(NewBB);
 
-    std::vector<NodeRef> PredBlocks;
+    SmallVector<NodeRef, 4> PredBlocks;
     for (auto Pred : children<Inverse<N>>(NewBB))
       PredBlocks.push_back(Pred);