[mlir][Linalg] Add an interface to decompose complex ops

This patch adds an interface, named AggregatedOpInterface, that decomposes
complex operations into simpler ones.

For now, make the interface specific to Linalg because although the concept
is general, the way to materialize it needs some maturing.

Use that interface with the softmax operator.

Differential Revision: https://reviews.llvm.org/D154363