This change updates PPC64.rules to recognize constant shift
counts and generate more efficient code sequences in those cases.

Fixes #17336

Change-Id: I8a7b812408d7a68388df41e42bad045dd214be17
Reviewed-on: https://go-review.googlesource.com/30310
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>