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/V3Cfg.cpp

@@ -115,6 +115,7 @@ void CfgGraph::rpoBlocks() {
     // Assign edge IDs
     size_t edgeCount = 0;
     for (V3GraphVertex& v : vertices()) {
+        // cppcheck-suppress constVariableReference // cppcheck is wrong
         for (V3GraphEdge& e : v.outEdges()) static_cast<CfgEdge&>(e).m_id = edgeCount++;
     }
     UASSERT_OBJ(edgeCount == m_nEdges, m_enterp, "Inconsistent edge count");