r600/sfn: Don't try to re-use iterators when the set is made empty

From Gert Wollny
c13de0509c43f9b9764dc939aa64fe70c6a80870 in mainline Mesa

fixes games/xonotic crash on r600 reported by edd@ on bugs@

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

@@ -373,7 +373,11 @@ CopyPropFwdVisitor::visit(AluInstr *instr)
    auto ii = dest->uses().begin();
    auto ie = dest->uses().end();
 
-   while(ii != ie) {
+   /** libc++ seems to invalidate the end iterator too if a std::set is
+    *  made empty by an erase operation,
+    *  https://gitlab.freedesktop.org/mesa/mesa/-/issues/7931
+    */
+   while(ii != ie && !dest->uses().empty()) {
       auto i = *ii;
       ++ii;
       /* SSA can always be propagated, registers only in the same block