[MLIR][Standard] Make the `dim` operation index an operand.

[MLIR][Standard] Make the `dim` operation index an operand.

Allow for dynamic indices in the `dim` operation.
Rather than an attribute, the index is now an operand of type `index`.
This allows to apply the operation to dynamically ranked tensors.
The correct lowering of dynamic indices remains to be implemented.

Differential Revision: https://reviews.llvm.org/D81551