[mlir][Linalg] Refactor in preparation for automatic Linalg "named" ops.

[mlir][Linalg] Refactor in preparation for automatic Linalg "named" ops.

This revision prepares the ground for declaratively defining Linalg "named" ops.
Such named ops form the backbone of operations that are ubiquitous in the ML
application domain.

This revision closely related to the definition of a "Tensor Computation
Primitives Dialect" and demonstrates that ops can be expressed as declarative
configurations of the `linalg.generic` op.

Differential Revision: https://reviews.llvm.org/D74491