Avoid hardcoded 4096 constant

This commit creates a static constexpr limit for the IntegerType
bitwidth and uses it. The check had to be moved because Token is
not aware of IR/Type and it was a sign the abstraction leaked:
bitwidth limit is not a property of the Token but of the IntegerType.

Added a positive and a negative test at the limit.

PiperOrigin-RevId: 210388192