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/V3Const.cpp

@@ -3145,7 +3145,7 @@ class ConstVisitor final : public VNVisitor {
         if (nodep->timingControlp()) m_hasJumpDelay = true;
         if (m_doNConst && replaceNodeAssign(nodep)) return;
     }
-    void visit(AstAssignAlias* nodep) override {
+    void visit(AstAlias* nodep) override {
         // Don't perform any optimizations, keep the alias around
     }
     void visit(AstAssignVarScope* nodep) override {