coccinelle: deref_null: improve performance

Move rules looking for some special cases of safe dereferences before
the collection of NULL-tested values.  The special cases are fairly
rare, but somewhat costly to find, because isomorphisms create many
variants of the rules.  There is thus no need to search for them over
and over for each NULL tested expression.  Collecting them just once
is sufficient and more efficient.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>