var x uint64
uint8(x >> 56)

We don't need to generate any code for the uint8().

Update #15090

Change-Id: Ie1ca4e32022dccf7f7bc42d531a285521fb67872
Reviewed-on: https://go-review.googlesource.com/28191
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>