[mlir] Add in-dialect lowering of gpu.all_reduce.

Reviewers: ftynse, nicolasvasilache, herhut

Reviewed By: ftynse, herhut

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72129