Old buggy hardware incorrectly executes the shift-left-K
then shift-right-K idiom for clearing K leftmost bits.
Use a right rotate instead of shift to avoid triggering the
bug.

Fixes #19809.

Change-Id: I6dc646b183c29e9d01aef944729f34388dcc687d
Reviewed-on: https://go-review.googlesource.com/39310
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>