[mlir][Python] use canonical Python `isinstance` instead of `Type.isinstance` (#172892)
We've been able to do `isinstance(x, Type)` for a quite a while now
(since
https://github.com/llvm/llvm-project/commit/bfb1ba752655bf09b35c486f6cc9817dbedfb1bb)
so remove `Type.isinstance` and the the special-casing
(`_is_integer_type`, `_is_floating_point_type`, `_is_index_type`) in
some places (and therefore support various `fp8`, `fp6`, `fp4` types).