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/V3DfgColorSCCs.cpp

@@ -55,7 +55,7 @@ class ColorStronglyConnectedComponents final {
         const size_t rootIndex = vtxState.index = ++m_index;
 
         // Visit children
-        vtx.forEachSink([&](DfgVertex& child) {
+        vtx.foreachSink([&](DfgVertex& child) {
             VertexState& childSatate = child.user<VertexState>();
             // If the child has not yet been visited, then continue traversal
             if (childSatate.index == UNASSIGNED) visitColorSCCs(child, childSatate);
@@ -63,6 +63,7 @@ class ColorStronglyConnectedComponents final {
             if (childSatate.component == UNASSIGNED) {
                 if (vtxState.index > childSatate.index) vtxState.index = childSatate.index;
             }
+            return false;
         });
 
         if (vtxState.index == rootIndex) {
@@ -73,7 +74,7 @@ class ColorStronglyConnectedComponents final {
                                    || m_stack.back()->getUser<VertexState>().index < rootIndex;
             // We also need a separate component for vertices that drive themselves (which can
             // happen for input like 'assign a = a'), as we want to extract them (they are cyclic).
-            const bool drivesSelf = vtx.findSink<DfgVertex>([&vtx](const DfgVertex& sink) {  //
+            const bool drivesSelf = vtx.foreachSink([&](const DfgVertex& sink) {  //
                 return &vtx == &sink;
             });
 
@@ -107,7 +108,7 @@ class ColorStronglyConnectedComponents final {
         for (DfgVertexVar& vtx : m_dfg.varVertices()) {
             VertexState& vtxState = vtx.user<VertexState>();
             // If it has no input or no outputs, it cannot be part of a non-trivial SCC.
-            if (vtx.arity() == 0 || !vtx.hasSinks()) {
+            if ((!vtx.srcp() && !vtx.defaultp()) || !vtx.hasSinks()) {
                 UDEBUGONLY(UASSERT_OBJ(vtxState.index == UNASSIGNED || vtxState.component == 0,
                                        &vtx, "Non circular variable must be in a trivial SCC"););
                 vtxState.index = 0;