Iternals: Fix AstSplitPlaceholder, it is a statement (#6280) (#6309)

src/V3AstNodeOther.h

@@ -1616,13 +1616,6 @@ public:
     bool hasCombo() const;  // Includes a COMBO SenItem
     bool hasHybrid() const;  // Includes a HYBRID SenItem
 };
-class AstSplitPlaceholder final : public AstNode {
-public:
-    // Dummy node used within V3Split; never exists outside of V3Split.
-    explicit AstSplitPlaceholder(FileLine* fl)
-        : ASTGEN_SUPER_SplitPlaceholder(fl) {}
-    ASTGEN_MEMBERS_AstSplitPlaceholder;
-};
 class AstStrengthSpec final : public AstNode {
     VStrength m_s0;  // Drive 0 strength
     VStrength m_s1;  // Drive 1 strength

src/V3AstNodeStmt.h

@@ -815,6 +815,13 @@ public:
     ASTGEN_MEMBERS_AstSetuphold;
     bool sameNode(const AstNode* /*samep*/) const override { return true; }
 };
+class AstSplitPlaceholder final : public AstNodeStmt {
+public:
+    // Dummy node used within V3Split; never exists outside of V3Split.
+    explicit AstSplitPlaceholder(FileLine* fl)
+        : ASTGEN_SUPER_SplitPlaceholder(fl) {}
+    ASTGEN_MEMBERS_AstSplitPlaceholder;
+};
 class AstStackTraceT final : public AstNodeStmt {
     // $stacktrace used as task
 public: