Internals: Improve DFG implementation details (#6355)

Large scale refactoring to simplify some of the more obtuse internals of
DFG. Remove multiple redundant internal APIs, simplify representation of
variables, fix potential unsoundness in circular decomposition. No
functional change intended.

src/V3DfgOptimizer.cpp

@@ -324,7 +324,7 @@ class DataflowOptimize final {
 
         // Mark interfaces that might be referenced by a virtual interface
         if (v3Global.hasVirtIfaces()) {
-            netlistp->typeTablep()->foreach([](AstIfaceRefDType* nodep) {
+            netlistp->typeTablep()->foreach([](const AstIfaceRefDType* nodep) {
                 if (!nodep->isVirtual()) return;
                 nodep->ifaceViaCellp()->setHasVirtualRef();
             });