nir/opt_remove_phis: handle phis with no sources

This can happen with loops with unreachable exits which are later
optimized away.

Fixes assertion in dEQP-VK.graphicsfuzz.unreachable-loops with RADV.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 12372d60)