[SILOptimizer] Projection tree can vend users.

Added getUsers to ProjectionTree.  The new method vands, via an out
parameter, a set of all the users of all of the nodes in the projection
tree that are themselves not in the projection tree by way of
getNonProjUsers.  Took this opportunity to tweak getNonProjUsers to vend
a const ArrayRef rather than a SmallVector.

Excerpted from @GottesM's #16756.

Author: nate-chandler

include/swift/SIL/Projection.h

@@ -757,9 +757,9 @@ class ProjectionTreeNode {
 
   llvm::Optional<Projection> &getProjection() { return Proj; }
 
-  llvm::SmallVector<Operand *, 4> getNonProjUsers() const {
-    return NonProjUsers;
-  };
+  const ArrayRef<Operand *> getNonProjUsers() const {
+    return llvm::makeArrayRef(NonProjUsers);
+  }
 
   bool isLeaf() const { return ChildProjections.empty(); }
 
@@ -960,7 +960,9 @@ class ProjectionTree {
   void
   replaceValueUsesWithLeafUses(SILBuilder &B, SILLocation Loc,
                                llvm::SmallVectorImpl<SILValue> &Leafs);
- 
+
+  void getUsers(SmallPtrSetImpl<SILInstruction *> &users) const;
+
 private:
   void createRoot(SILType BaseTy) {
     assert(ProjectionTreeNodes.empty() &&

lib/SIL/Projection.cpp

@@ -1514,3 +1514,11 @@ replaceValueUsesWithLeafUses(SILBuilder &Builder, SILLocation Loc,
     NewNodes.clear();
   }
 }
+
+void ProjectionTree::getUsers(SmallPtrSetImpl<SILInstruction *> &users) const {
+  for (auto *node : ProjectionTreeNodes) {
+    for (auto *op : node->getNonProjUsers()) {
+      users.insert(op->getUser());
+    }
+  }
+}