[BUG] rebind being too lenient

[soraros]

Bug description

Type equity is not always guaranteed. @lattner suggested on Discord that it's not the intended behaviour.

Steps to reproduce

fn main():
  a = Int64(0)
  b = rebind[UInt64](a)  # compiles 

System information

Mojo 2024.9.2905 (d72d1fbc) on Docker, Intel Mac

[lattner]

Hrm, interesting issue. The problem is that we are lowering away declared type information /before/ we get to instantiation. This means that Int and UInt end up looking like "just a bag of i64". It isn't great that we allow this, but probably not the end of the world. @Mogball just fyi