[TIR] Improve Let/LetStmt support. (#5949)

[TIR] Improve Let/LetStmt support. (#5949)

Let/LetStmt are useful primitives to create variable bindings.
While let binding are harmful for simplification and integer analysis,
they are useful for other cases:

- C0: LetStmt is useful to represent a step that has side effect(e.g. call a PRNG)
- C1: Let expression can be used to create deep nested expression for complicated functions.

This PR improves the let support in the following ways:
- Enable vectorization support for let
- Change let simplification strategy to simplify the most trivial case
  while ignore more complicated cases(to avoid deep nest explosion)
- Enhance arith module to handle const bound and modular set for let.

The overall recommendation is to only use Let in the cases when necessary(C0, C1).