[mlir] [VectorOps] Framework for progressive lowering of vector.contract

Summary:
Lowers all free/batch dimensions in a vector.contract progressively
into simpler vector.contract operations until a direct vector.reduction
operation is reached. Then lowers 1-D reductions into vector.reduce.

Still TBD:
multi-dimensional contractions that remain after removing all the parallel dims

Reviewers: nicolasvasilache, andydavis1, rriddle

Reviewed By: andydavis1

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74797