cmd/compile already optimizes "string(b) == s" to avoid allocating a
temporary string.

Change-Id: I4244fbeae8d350261494135c357f9a6e2ab7ace3
Reviewed-on: https://go-review.googlesource.com/28931
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>