[MLIR] Remove all-reduce lowering from GPU to NVVM. Use in-dialect lowering instead.

Reviewers: herhut, mravishankar

Reviewed By: herhut

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73794