nir/copy_prop_vars: prefer using entries from equal derefs

nir/copy_prop_vars: prefer using entries from equal derefs

When looking up an entry to use, always prefer an equal match, as it
more likely to contain reusable SSA or derefs to propagate.

This will be necessary when adding entries with array derefs of
vectors, because we don't want the vector if the equal entry (an array
deref of that vector) is present.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>