Use (-x)>>63 instead of ((x-1)>>63)^-1 to get a mask that
is 0 when x is 0 and all ones when x is positive.

Saves one instruction when slicing.

Change-Id: Ib46d53d3aac6530ac481fa2f265a6eadf3df0567
Reviewed-on: https://go-review.googlesource.com/35641
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>