In f the extra & 63 is redundant because SHRQ already
looks at the bottom 6 bits only. This is a trick on AMD64
to get rid of CMPQ/SBBQ/ANDQ if one knows that the shift
counter is small.

func f(x uint64, s uint) uint64 {
        return x >> (s & 63)
}

Change-Id: I4861c902168dabec9a6a14a85750246dde94fc08
Reviewed-on: https://go-review.googlesource.com/21073Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>