Support more complex combinational assignments in DFG. (#6205)

Previously DFG was limited to having a Sel, or an ArraySel potentially
under a Concat on the LHS of combinational assignments. Other forms or
combinations were not representable in the graph.

This adds support for arbitrary combinations of the above by
combining DfgSplicePacked and DfgSpliceArray vertices introduced in
 #6176. In particular, Sel(ArraySel(VarRef,_),_) enables a lot more code
to be represented in DFG.

src/V3DfgRegularize.cpp

@@ -50,7 +50,7 @@ class DfgRegularize final {
                 if (vtx.is<DfgVertexSplice>()) {
                     const bool hasNonVarSink
                         = vtx.findSink<DfgVertex>([](const DfgVertex& snk) {  //
-                              return !snk.is<DfgVertexVar>();
+                              return !snk.is<DfgVertexVar>() && !snk.is<DfgVertexSplice>();
                           });
                     return hasNonVarSink;
                 }