[mlir] [VectorOps] Multi-dim reductions for lowering vector.contract

Summary:
This implements the last step for lowering vector.contract progressively
to LLVM IR (except for masks). Multi-dimensional reductions that remain
after expanding all parallel dimensions are lowered into into simpler
vector.contract operations until a trivial 1-dim reduction remains.

Reviewers: nicolasvasilache, andydavis1

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/D74880