Convert linalg.generic to linalg.matmul

Input: linalg.generic representing matmul (https://github.com/intel/mlir-extensions/blob/refactor/test/PlaidML/OpTest.Dot.mlir)
Output: linalg.matmul (https://github.com/intel/mlir-extensions/blob/refactor/test/PlaidML/OpTest.Matmul.mlir)

Motivation:
We are using GML_ST (an MHLO dialect) to do tiling of larger GEMM into smaller ones. However, GML_ST only works on linalg.generic. However, once the tiling and kernel outlining is done with GML_ST, we would like the smaller gemm (represented in linalg.generic) to transform to linalg.matmul. This would give us the ability to directly transform the linalg.matmul to spirv in a easy way. It's always good to have named op than generic to lower from.