[MLIR][Python] Use ir.Value directly instead of _SubClassValueT (#82341)

_SubClassValueT is only useful when it is has >1 usage in a signature.
This was not true for the signatures produced by tblgen.

For example

def call(result, callee, operands_, *, loc=None, ip=None) ->
_SubClassValueT:
        ...

here a type checker does not have enough information to infer a type
argument for _SubClassValueT, and thus effectively treats it as Any.