spirv: Skip phis in unreachable blocks in the second phi pass

Closes: #3253
Fixes: 22fdb2f8551 "nir/spirv: Update to the latest revision"
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5855>
1 file changed