Contracts/insert_before_and_update_jumps: only update jump edge

The incoming edge from a `goto` instruction may also be the
non-branching case, which must not result in redirecting this goto.

regression/contracts-dfcc/loop_contracts_do_while/assigns.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 assigns.c
 --dfcc main --apply-loop-contracts
 ^EXIT=0$

regression/contracts-dfcc/loop_contracts_do_while/side_effect.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 side_effect.c
 --dfcc main --apply-loop-contracts
 ^EXIT=0$

src/goto-instrument/contracts/utils.cpp

@@ -250,7 +250,7 @@ void insert_before_and_update_jumps(
   const auto new_target = destination.insert_before(target, i);
   for(auto it : target->incoming_edges)
   {
-    if(it->is_goto())
+    if(it->is_goto() && it->get_target() == target)
       it->set_target(new_target);
   }
 }