nir/algebraic: Loosen a restriction on variables

nir/algebraic: Loosen a restriction on variables

Previously, we would fail if a variable had an assigned but unknown bit
size X and we tried to assign it an actual bit size.  However, this is
ok because, at the time we do the search, the variable does have an
actual bit size and it will match X because of the NIR rules.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>