nir: Give end_block its own index

nir: Give end_block its own index

Since there's no particular reason for the index to be 0, choose an
index that is not used by other block.  This is convenient when we
store "per-block" data in an array AND look for the successors
data (e.g. any kind of backwards data-flow analysis).

v2: Add a note about end_block's index. (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>