Apparently the old backend needs NEG instruction having RegRead set,
even this instruction does not take a Reg field... I don't think SSA
uses this flag, so just leave it as it was. SSA is still happy.

Fix ARM64 build on https://build.golang.org/?branch=dev.ssa

Change-Id: Ia7e7f2ca217ddae9af314d346af5406bbafb68e8
Reviewed-on: https://go-review.googlesource.com/25302Reviewed-by: David Chase <drchase@google.com>