The compiler can generate better code for shifts bounded to be less than 32
and thereby known to be less than any register width.
See https://golang.org/cl/109776.

Change-Id: I0c4c9f0faafa065fce3c10fd328830deb92f9e38
Reviewed-on: https://go-review.googlesource.com/c/111735
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>