ANDs of constants whose only set bits are leading or trailing can be
rewritten as two shifts instead.  This is slightly faster for 32 or
64 bit operands.

Change-Id: Id5c1ff27e5a4df22fac67b03b9bddb944871145d
Reviewed-on: https://go-review.googlesource.com/19485
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>