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: #7931



Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <!20394>

src/gallium/drivers/r600/sfn/sfn_optimizer.cpp

@@ -360,7 +360,12 @@ CopyPropFwdVisitor::visit(AluInstr *instr)
    auto src = instr->psrc(0);
    auto dest = instr->dest();
 
-   for (auto& i : dest->uses()) {
+   auto ii = dest->uses().begin();
+   auto ie = dest->uses().end();
+
+   while(ii != ie) {
+      auto i = *ii;
+      ++ii;
       /* SSA can always be propagated, registers only in the same block
        * and only if they are assigned in the same block */
       bool can_propagate = dest->has_flag(Register::ssa);