nir/validate: Use a ralloc context for our temporary data

nir/validate: Use a ralloc context for our temporary data

All of our hash tables and sets are already using ralloc.  There's
really no good reason why we don't just make a ralloc context rather
than try to remember to clean everything up manually.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>