[release-branch.go1.7] cmd/compile: compare size in dead store elimination

This CL is a manual backpatch of CL 27320 into the 1.7.1 release branch.

The manual backpatch is required because OpZero changed from having a
size as its AuxInt to having a size+align as its AuxInt (that was to support
the ARM SSA backend).  Otherwise the CLs should be identical.

Please review carefully!

Change-Id: I569b759c06d1971c9c62dc5dd589abc7ef7c844a
Reviewed-on: https://go-review.googlesource.com/28670


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>