[release-branch.go1.10] 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>
(cherry picked from commit
1566bf90253097dfec83f621a48e761ded4d2144)
Reviewed-on: https://go-review.googlesource.com/94216