Internals: More const. No functional change intended.

src/V3Split.cpp

@@ -242,10 +242,10 @@ private:
     // AstVarScope::user2p      -> Var SplitNodeVertex* for delayed assignment var, 0=not set yet
     // Ast*::user3p             -> Statement SplitLogicVertex* (temporary only)
     // Ast*::user4              -> Current ordering number (reorderBlock usage)
-    AstUser1InUse m_inuser1;
-    AstUser2InUse m_inuser2;
-    AstUser3InUse m_inuser3;
-    AstUser4InUse m_inuser4;
+    const AstUser1InUse m_inuser1;
+    const AstUser2InUse m_inuser2;
+    const AstUser3InUse m_inuser3;
+    const AstUser4InUse m_inuser4;
 
 protected:
     // STATE