nir/opt_if: Re-materialize derefs in use blocks before peeling loops

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107879
Cc: "18.2" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 864c7805)