[release-branch.go1.9] cmd/compile: fix evaluation of "" < s
    
    Fixes #24934
    
    Change-Id: Ifa79ab3dfe69297eeef85f7193cd5f85e5982bc5
    Reviewed-on: https://go-review.googlesource.com/106655
    Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Matthew Dempsky <mdempsky@google.com>
    Reviewed-on: https://go-review.googlesource.com/108943
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>