[MLIR] add linalg.argmax and linalg.argmin

Hello,

I wonder if it is possbile to add linalg.argmax and linalg.argmin to better describe such computation rather than making it as a series of selection and compare.

The issue proposes adding linalg.argmax and linalg.argmin but names no files, tests, entry points, or acceptance criteria. Start by investigating how the current series of selection and compare operations represents this computation, then define the operation semantics and tests needed to establish completion.

Written by the indexing model from the issue text.