[TIRx] Generalize expression functor signatures

[tqchen]

Expression unification gives TIRX a shared Expr surface, but its visitor and mutator APIs still expose primitive-only signatures. That mismatch prevents general expressions from flowing through the existing traversal structure and leaves statement traversal with overlapping customization hooks.

This refactor generalizes the existing ExprFunctor, ExprVisitor, and ExprMutator signatures in place to accept and return Expr. Statement visitors and mutators expose a single virtual VisitExpr(const Expr&) hook, while primitive statement reconstruction uses a non-virtual checked VisitPrimExpr helper so invalid narrowing fails at the boundary. Public pre-order and post-order traversal entry points accept general Expr roots.

The existing specialization, vtable, dispatch registration, and class structure remain intact; the change adds no parallel functor, fallback dispatcher, or alternate implementation path.

[gemini-code-assist]

Code Review

This pull request removes the SizeVar and SizeVarNode classes, unifying symbolic variables under Var and VarNode. It also updates ExprFunctor and its subclasses to operate on Expr instead of PrimExpr to allow visiting and mutating general expressions. Additionally, BlockBuilder in Relax is updated to track active scopes and blocks to safely unwind them on exceptional exits. The reviewer identified a potential issue in _exit_function_scope where exceptions during block or scope finalization could skip subsequent cleanup steps, and suggested using a try...finally block to ensure robust cleanup.

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