Internals: Manage DfgVertex user data via a typed map (#6356)

Add DfgUserMap as a handle around the one pointer worth of algorithm
specific 'user' storage in each DfgVertex. This reduces verbosity,
improves type safety and correctness. Also enables us to remove one
pointer from DfgVertex to reduce memory use. No functional change.

src/V3DfgSynthesize.cpp

@@ -1822,8 +1822,8 @@ void V3DfgPasses::synthesize(DfgGraph& dfg, V3DfgContext& ctx) {
         // Otherwise figure out which vertices are worth synthesizing.
 
         // Find cycles
-        const auto userDataInUse = dfg.userDataInUse();
-        V3DfgPasses::colorStronglyConnectedComponents(dfg);
+        DfgUserMap<uint64_t> scc = dfg.makeUserMap<uint64_t>();
+        V3DfgPasses::colorStronglyConnectedComponents(dfg, scc);
 
         // First, gather variables, we will then attempt to synthesize all their drivers
         std::vector<DfgVertexVar*> varps;
@@ -1832,7 +1832,7 @@ void V3DfgPasses::synthesize(DfgGraph& dfg, V3DfgContext& ctx) {
             if (!var.srcp()) continue;
 
             // Circular variable - synthesize
-            if (var.getUser<uint64_t>()) {
+            if (scc.at(var)) {
                 varps.emplace_back(&var);
                 continue;
             }