spirv: Skip phis in unreachable blocks in the second phi pass
Closes: #3253 Fixes: 22fdb2f8 "nir/spirv: Update to the latest revision" Reviewed-by:Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com> Part-of: <mesa/mesa!5855> (cherry picked from commit 81773b4b)