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/Verilator.cpp

@@ -337,6 +337,7 @@ static void process() {
             V3Const::constifyAll(v3Global.rootp());
 
             // Flatten hierarchy, creating a SCOPE for each module's usage as a cell
+            // No more AstAlias after linkDotScope
             V3Scope::scopeAll(v3Global.rootp());
             V3LinkDot::linkDotScope(v3Global.rootp());