[mlir] Add primitive transform pattern to rewrite linalg.fill into vector.broadcast form.

Summary:
This diff adds a transformation patter to rewrite linalg.fill as broadcasting a scaler into a vector.
It uses the same preconditioning as matmul (memory is contiguous).

Reviewers: nicolasvasilache

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73391