[dev.ssa] cmd/compile: implement OSTRUCTLIT and OARRAYLIT

[dev.ssa] cmd/compile: implement OSTRUCTLIT and OARRAYLIT

The frontend rewrites most literals, so we see only zero
ones during SSA construction.  We can implement those
using the existing zeroing behavior.

Change-Id: I390ad1be0a4b6729baf0c8936c7610aae2aef049
Reviewed-on: https://go-review.googlesource.com/14754
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>