nir/spirv: skip unreachable blocks in Phi second pass

nir/spirv: skip unreachable blocks in Phi second pass

Only the blocks that are reachable are inserted with an end_nop
instruction at the end.

When handling the Phi second pass, if the Phi has a parent block that
does not have an end_nop then it means this block is unreachable, and
thus we can ignore it, as the Phi will never come through it.

Fixes dEQP-VK.graphicsfuzz.uninit-element-cast-in-loop.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>