[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