[MLIR] Move AtomicRMW into MemRef dialect and enum into Arith

Per the discussion in https://reviews.llvm.org/D116345 it makes sense
to move AtomicRMWOp out of the standard dialect. This was accentuated by the
need to add a fold op with a memref::cast. The only dialect
that would permit this is the memref dialect (keeping it in the standard dialect
or moving it to the arithmetic dialect would require those dialects to have a
dependency on the memref dialect, which breaks linking).

As the AtomicRMWKind enum is used throughout, this has been moved to Arith.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D116392