[release-branch.go1.10] cmd/compile: fix constant folding of right shifts
The sub-word shifts need to sign-extend before shifting, to avoid
bringing in data from higher in the argument.
Fixes #23812
Change-Id: I0a95a0b49c48f3b40b85765bb4a9bb492be0cd73
Reviewed-on: https://go-review.googlesource.com/93716
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit
755b36aa532f6b23081bf5eaf83449c1a6dd8114)
Reviewed-on: https://go-review.googlesource.com/94215
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>