[JITLink] Add comments, rename types for visitExistingEdges utility.

The "Fixers" name was a hangover from an earlier draft of the patch. "Visitors"
fits the function name(s).

Author: lhames

llvm/include/llvm/ExecutionEngine/JITLink/JITLink.h

@@ -1709,28 +1709,34 @@ Error markAllSymbolsLive(LinkGraph &G);
 Error makeTargetOutOfRangeError(const LinkGraph &G, const Block &B,
                                 const Edge &E);
 
-static inline void visitEdge(LinkGraph &G, Block *B, Edge &E) {}
-
-template <typename FixerT, typename... FixerTs>
-static void visitEdge(LinkGraph &G, Block *B, Edge &E, FixerT &&Fixer,
-                      FixerTs &&...Fixers) {
-  if (!Fixer.visitEdge(G, B, E))
-    visitEdge(G, B, E, std::forward<FixerTs>(Fixers)...);
+/// Base case for edge-visitors where the visitor-list is empty.
+inline void visitEdge(LinkGraph &G, Block *B, Edge &E) {}
+
+/// Applies the first visitor in the list to the given edge. If the visitor's
+/// visitEdge method returns true then we return immediately, otherwise we
+/// apply the next visitor.
+template <typename VisitorT, typename... VisitorTs>
+void visitEdge(LinkGraph &G, Block *B, Edge &E, VisitorT &&V,
+               VisitorTs &&...Vs) {
+  if (!V.visitEdge(G, B, E))
+    visitEdge(G, B, E, std::forward<VisitorTs>(Vs)...);
 }
 
-/// Visits edges exist in graph by Fixers.
+/// For each edge in the given graph, apply a list of visitors to the edge,
+/// stopping when the first visitor's visitEdge method returns true.
 ///
-/// Note: that if a fixer fixes the edge successfully,
-/// the rest of the fixers will not visit this edge.
-template <typename... FixerTs>
-void visitExistingEdges(LinkGraph &G, FixerTs &&...Fixers) {
-  // We're going to be adding new blocks, but we don't want to iterate over
-  // the new ones, so build a worklist.
+/// Only visits edges that were in the graph at call time: if any visitor
+/// adds new edges those will not be visited. Visitors are not allowed to
+/// remove edges (though they can change their kind, target, and addend).
+template <typename... VisitorTs>
+void visitExistingEdges(LinkGraph &G, VisitorTs &&...Vs) {
+  // We may add new blocks during this process, but we don't want to iterate
+  // over them, so build a worklist.
   std::vector<Block *> Worklist(G.blocks().begin(), G.blocks().end());
 
   for (auto *B : Worklist)
     for (auto &E : B->edges())
-      visitEdge(G, B, E, std::forward<FixerTs>(Fixers)...);
+      visitEdge(G, B, E, std::forward<VisitorTs>(Vs)...);
 }
 
 /// Create a LinkGraph from the given object buffer.