nir: Allocate predecessor and dominance frontier sets from block itself.

nir: Allocate predecessor and dominance frontier sets from block itself.

These sets are part of the block, and their lifetime needs to match the
block itself.  So, allocate them using the block itself as the context.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>