lima/ppir: add dummy op

lima/ppir: add dummy op

We can get following from NIR:

(1) r1 = r2
(2) r2 = ssa1

Note that r2 is read before it's assigned, so there's no node for
it in comp->var_nodes. We need to create a dummy node in this case
which sole purpose is to hold ppir_dest with reg in it.

Tested-by: Andreas Baierl <ichgeh@imkreisrum.de>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>