Optimize circular logic in Dfg (#7902)

Introduce a new DfgPrev vertex, representing the value of a variable
before any in-graph assignments. This can be used to break all remaining
cycles in the graph, so all Dfgs become acyclic after V3DfgBreakCycles.
The circular dataflow is still represented, and is taken care of by the
scheduler, it is just the DfgGraph that represents the logic that
becomes acyclic.

This makes V3DfgBreakCycles a mandatory transform, so drop the disabling
-fno-dfg-break-cycles option (still parsed, but has no effect).

Note the effect of this is small, as most cycles can be fixed up by
driver tracing, which is unchanged, but this is required for some
upcoming work.

src/V3Dfg.cpp

@@ -179,6 +179,40 @@ static void dumpDotVertex(std::ostream& os, const DfgVertex& vtx) {
         return;
     }
 
+    if (const DfgPrev* const prevVtxp = vtx.cast<DfgPrev>()) {
+        const AstVarScope* const vscp = prevVtxp->vscp();
+        os << toDotId(vtx);
+        // Begin attributes
+        os << " [";
+        // Begin 'label'
+        os << "label=\"";
+        // Name
+        os << vscp->prettyName();
+        // Address
+        os << '\n' << cvtToHex(prevVtxp);
+        // Type and fanout
+        os << '\n';
+        prevVtxp->dtype().astDtypep()->dumpSmall(os);
+        os << " / F" << prevVtxp->fanout();
+        // End 'label'
+        os << '"';
+        // Shape
+        if (prevVtxp->isPacked()) {
+            os << ", shape=box";
+        } else if (prevVtxp->isArray()) {
+            os << ", shape=box3d";
+        } else {
+            prevVtxp->v3fatalSrc("Unhandled variable type");
+        }
+        // Color
+        const char* const colorp = "mediumorchid1";  // Purple
+        os << ", style=filled";
+        os << ", fillcolor=\"" << colorp << "\"";
+        // End attributes
+        os << "]\n";
+        return;
+    }
+
     if (const DfgConst* const constVtxp = vtx.cast<DfgConst>()) {
         const V3Number& num = constVtxp->num();
 
@@ -479,6 +513,10 @@ void DfgVertex::typeCheck(const DfgGraph& dfg) const {
         CHECK(!v.srcp() || v.srcp()->dtype() == v.dtype(), "'srcp' should match");
         return;
     }
+    case VDfgType::Prev: {
+        CHECK(isPacked() || isArray(), "Should be Packed or Array type");
+        return;
+    }
     case VDfgType::SpliceArray:
     case VDfgType::SplicePacked: {
         const DfgVertexSplice& v = *as<DfgVertexSplice>();