nir/serialize: store 32-bit object IDs instead of 64-bit

nir/serialize: store 32-bit object IDs instead of 64-bit

That means we have only 30 bits for object IDs, because 2 bits are
sometimes used for something else.

This decrease the uncompressed shader size for the biggest Borderlands 2
shader from 33.6 KB to 23.2 KB. (31% decrease)

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>