[mlir] [VectorOps] Implement vector.reduce operation

Summary:
This new operation operates on 1-D vectors and
forms the bridge between vector.contract and
llvm intrinsics for vector reductions.

Reviewers: nicolasvasilache, andydavis1, ftynse

Reviewed By: nicolasvasilache

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/D74370