nv50/ir: fix emission of address reg in 3rd source
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91056 Signed-off-by:Ilia Mirkin <imirkin@alum.mit.edu> Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org> (cherry picked from commit d5f1253b)