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/V3DfgRegularize.cpp

@@ -63,7 +63,7 @@ class DfgRegularize final {
             // This is an op that requires a result variable. Ensure it is
             // assigned to one, and redirect other sinks read that variable.
             if (DfgVertexVar* const varp = vtx.getResultVar()) {
-                varp->sourceEdge<0>()->unlinkSource();
+                varp->srcp(nullptr);
                 vtx.replaceWith(varp);
                 varp->srcp(&vtx);
             } else {