nir/dead_cf: Stop relying on liveness analysis

nir/dead_cf: Stop relying on liveness analysis

The liveness analysis pass is fairly expensive because it has to build
large bit-sets and run a fix-point algorithm on them.  Instead of
requiring liveness for detecting if values escape a CF node, just take
advantage of the structured nature of NIR and use block indices instead.
This only requires the block index metadata which is the fastest we have
metadata to generate.

No shader-db changes on Kaby Lake

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>