[mlir] Simplify a few cast implementations

`{Attribute,Type}::classof` are never actually called at runtime due to
the early exit in their `CastInfo` implementations. By using `if
constexpr` we can avoid needing to define them.

We also don't need to check `is_same_v` here, since this is already
covered by `is_base_of_v`.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D142863