nir/serialize: Alloc constants off the variable

nir_sweep assumes that constants area always allocated off the variable
to which they belong.  Violating this assumption causes them to get
freed early and leads to use-after-free bugs.

Fixes: 120da009 "nir: add serialization and deserialization"
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107366


Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>
(cherry picked from commit f214baf7)