Internal: Refactor AstAssignAlias (#6280) (#6473)

Rename AstAssignAlias to AstAlias and make it derive from AstNode
instead of AstNodeStmt.

Replace AstAlias with AstAssignW in V3LinkDot::linkDotScope, which is
the last place we need to be aware of the alias construct. Using
AstAssignW dowstream enables further optimization while preserving the
same functionality.

src/V3Slice.cpp

@@ -286,7 +286,6 @@ class SliceVisitor final : public VNVisitor {
     void visit(AstNodeAssign* nodep) override {
         // Called recursively on newly created assignments
         if (nodep->user1SetOnce()) return;  // Process once
-        if (VN_IS(nodep, AssignAlias)) return;
         UINFOTREE(9, nodep, "", "Deslice-In");
         VL_RESTORER(m_assignError);
         VL_RESTORER(m_assignp);