Optimize CReset in Dfg (#7737)

Teach DFG about CReset. This is not so much to optimize CReset itself, but to enable synthesizing logic involving CReset, which does appear with automatic variables used only in certain branches

src/V3DfgColorSCCs.cpp

@@ -125,6 +125,12 @@ class ColorStronglyConnectedComponents final {
 
         // Initialize state of operation vertices
         for (const DfgVertex& vtx : m_dfg.opVertices()) {
+            // If it has no inputs or no outputs, it cannot be part of a non-trivial SCC.
+            if (!vtx.nInputs() || !vtx.hasSinks()) {
+                index(vtx) = 0;
+                component(vtx) = 0;
+                continue;
+            }
             index(vtx) = UNASSIGNED;
             component(vtx) = UNASSIGNED;
         }