nir: Allocate register fields out of the register itself.

nir: Allocate register fields out of the register itself.

The lifetime of each register's use/def/if_use sets needs to match the
register itself.  So, allocate them using the register itself as the
context.

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