[mlir][linalg] Start a named ops to generic ops pass

This commit starts a new pass and patterns for converting Linalg
named ops to generic ops. This enables us to leverage the flexbility
from generic ops during transformations. Right now only linalg.conv
is supported; others will be added when useful.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D91357