Optimize conditional merging across some impure statements (#7159)

- Allow reordering pure statements with DPI import calls iff no public
  variables (including those read via a DPI export) are involved. This
  ensures the DPI import can't observe the reordering
- Allow reordering of pure statements with AstDisplay and AstStop. This
  requires an assumption that AstDisplay and AstStop will not read or
  write model state other than via a VarRef explicitly present int the
  Ast.
Overall this allows eliminating a lot of conditionals around assertions,
which were previously not possible.

src/V3Task.cpp

@@ -765,7 +765,8 @@ class TaskVisitor final : public VNVisitor {
             // __Vscopep
             ccallp->addArgsp(snp);
             // __Vfilenamep
-            ccallp->addArgsp(new AstCExpr{flp, "\"" + flp->filenameEsc() + "\"", 64});
+            ccallp->addArgsp(
+                new AstCExpr{flp, AstCExpr::Pure{}, "\"" + flp->filenameEsc() + "\"", 64});
             // __Vlineno
             ccallp->addArgsp(new AstConst(flp, flp->lineno()));
         }