Internals: Add cloneTreePure to prepare for side effect check.

Use cloneTreePure when what is being cloned must be side-effect free.
Use cloneTree when safe to contain side effects (e.g. cloning module).

src/V3Delayed.cpp

@@ -353,7 +353,7 @@ private:
         }
         if (bitselp) {
             selectsp = new AstSel{nodep->fileline(), selectsp, bitreadp,
-                                  bitselp->widthp()->cloneTree(false)};
+                                  bitselp->widthp()->cloneTreePure(false)};  // Always pure
         }
         // Build "IF (changeit) ...
         UINFO(9, "   For " << setvscp << endl);