Old backends did not implement them, but SSA do.

Change-Id: I543b2281dcf4bab0da37c9b1f26a5ef55a0ea11b
Reviewed-on: https://go-review.googlesource.com/29278
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>