r600/sfn: Don't try to re-use the iterator when uses is updated
With libc++ the iterator is invalidated when an element is removed from the set, so open code the loop instead of using a range based for to ensure that no invalidated iterator is used.
Fixes: f3415c (r600/sfn: copy propagate register load chains)
Closes: #7931 (closed)
Edited by Gert Wollny