r600/sfn: Don't try to re-use the iterator when uses is updated

It seems on libc++ the iterator is invalidated when an element is removed
from the set, so make sure that we don't implicitely use the old,
invalidated iterator in the range based - open code the loop using while
instead.

Fixes: f3415c (r600/sfn: copy propagate register load chains)

Closes: mesa/mesa#7931



Signed-off-by: Gert Wollny <gert.wollny@collabora.com>