nv50/ir: make sure cfg iterator always hits all blocks

nv50/ir: make sure cfg iterator always hits all blocks

In some very specially-crafted cases, we could attempt to visit a node
that has already been visited, and then run out of bb's to visit, while
there were still cross blocks on the list. Make sure that those get
moved over in that case.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96274
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 092f994a034b718f6170c631655ccaba8d2aa768)