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.
This can happen with loops with unreachable exits which are later optimized away.
Fixes assertion in dEQP-VK.graphicsfuzz.unreachable-loops with RADV.