[mlir][Linalg] Add support to lower named ops to loops.

This revision adds support to allow named ops to lower to loops.
Linalg.batch_matmul successfully lowers to loops and to LLVM.

In the process, this test also activates linalg to affine loops.
However padded convolutions to not lower to affine.load atm so this revision overrides the type of underlying load / store operation.

Differential Revision: https://reviews.llvm.org/D79135