[mlir][Linalg] Adding support for linalg_matmul with tensors.

[mlir][Linalg] Adding support for linalg_matmul with tensors.

Summary:
This revision provides 2 versions of matmul with tensors to account for the differences in buffer vs value semantics:
1. `C(i, j) = sum_{r_k} A(i, r_k) * B(r_k, j)`
2. `D(i, j) = C(i, j) + sum_{r_k} A(i, r_k) * B(r_k, j)`

Reviewers: ftynse

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73796