Atomic add rules were depending on CSE to combine duplicate atomic ops.
With -N, CSE doesn't run.

Redo the rules for atomic add so there's only one atomic op.
Introduce an add-to-first-part-of-tuple pseudo-ops to make the atomic add result correct.

Change-Id: Ib132247051abe5f80fefad6c197db8df8ee06427
Reviewed-on: https://go-review.googlesource.com/27991
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>