Optimize constify within Expand and Subst (#6111)

These passes blow up the Ast size on some designs, so delaying running V3Const
until after the whole pass can notably increase peak memory usage. In this
patch we apply V3Const per CFunc within these passes, which saves on memory.
Added -fno-const-eager to disable the intra-pass V3Const application, for
debugging.

src/V3Expand.cpp

@@ -357,6 +357,16 @@ class ExpandVisitor final : public VNVisitor {
     }
 
     // VISITORS
+    void visit(AstCFunc* nodep) override {
+        iterateChildren(nodep);
+
+        // Constant fold here, as Ast size can likely be reduced
+        if (v3Global.opt.fConstEager()) {
+            AstNode* const editedp = V3Const::constifyEditCpp(nodep);
+            UASSERT_OBJ(editedp == nodep, editedp, "Should not have replaced CFunc");
+        }
+    }
+
     void visit(AstExtend* nodep) override {
         if (nodep->user1SetOnce()) return;  // Process once
         iterateChildren(nodep);