[mlir] Move std.generic_atomic_rmw to the memref dialect

This is part of splitting up the standard dialect. The move makes sense anyways,
given that the memref dialect already holds memref.atomic_rmw which is the non-region
sibling operation of std.generic_atomic_rmw (the relationship is even more clear given
they have nearly the same description % how they represent the inner computation).

Differential Revision: https://reviews.llvm.org/D118209