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