nir/validate: assume() that hashtable entry exists

nir/validate: assume() that hashtable entry exists

At this point, it would require a logic error in nir_validate to not
have already populated this hashtable entry, but coverity doesn't
realize that:

CID 1265547 (#1 of 1): Dereference null return value (NULL_RETURNS)3.
dereference: Dereferencing a null pointer entry.

CID 1271039 (#1 of 1): Dereference null return value (NULL_RETURNS)3.
dereference: Dereferencing a null pointer entry.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>