AMDGPU: Fix LiveVariables verifier error for values defined before SI_END_CF

GlobalISel happens to insert some constant materializes before SI_END_CF
in one test. These need to be excluded from AliveBlocks since they
are defined in the original block and used in the split block,
so they aren't fully alive through either block.

The case where the value defined in the first block which was originally used
in a later block is still broken.

Avoids a verifier error in a future patch.