[release-branch.go1.9] cmd/compile: fix constant folding of right shifts on s390x
    
    Repeat previous fix on amd64 for s390x.
    Sub-word right shifts should sign extend before shifting.
    
    Update #23812
    
    Change-Id: I2d770190c7d8a22310b0dbd9facb3fb05afa362a
    Reviewed-on: https://go-review.googlesource.com/94028
    Run-TryBot: Keith Randall <khr@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    Reviewed-on: https://go-review.googlesource.com/102777
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    Reviewed-by: Keith Randall <khr@golang.org>