Given the following situation:

```

>->- alive >->->->
                  \
                    >-> alive
                  /
   unreachable >->

```

We would attempt to codegen the dead block (all blocks ensure their in's are processed first).

However dEQP-VK.spirv_assembly.instruction.graphics.conditional_branch.same_labels_false_frag seems to have dead blocks that use values from blocks it couldn't possibly use (as it has no ins).

While this is probably just a broken test, it is still good to harden up the compiler to pathological cases like this.

Tests: dEQP-VK.spirv_assembly.instruction.compute.*
Tests: dEQP-VK.spirv_assembly.instruction.graphics.*
Bug: b/128527271
Change-Id: Ibaacf6a58cf7ab6e73771b58cfe67f0c394e53c6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27948Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>