Don't error out when alias analysis fails during loop assigns inference

Author: qinheping

regression/contracts/loop_assigns-fail/test.desc

@@ -1,10 +1,11 @@
 CORE
 main.c
 --apply-loop-contracts
-activate-multi-line-match
-Failed to infer variables being modified by the loop at file main.c line \d+ function main\.\nPlease specify an assigns clause\.\nReason:\nAlias analysis returned UNKNOWN!
-^EXIT=6$
+^EXIT=10$
 ^SIGNAL=0$
+^\[main.assigns.\d+\] .* Check that i is assignable: SUCCESS$
+^\[main.assigns.\d+\] .* Check that b->data\[(.*)i\] is assignable: FAILURE$
+^VERIFICATION FAILED$
 --
 --
 This test (taken from #6021) shows the need for assigns clauses on loops.

src/goto-instrument/loop_utils.cpp

@@ -52,7 +52,7 @@ void get_assigns_lhs(
       const typecast_exprt typed_mod{mod, ptr.pointer.type()};
       if(mod.id() == ID_unknown)
       {
-        throw analysis_exceptiont("Alias analysis returned UNKNOWN!");
+        continue;
       }
       if(ptr.offset.is_nil())
         assigns.insert(dereference_exprt{typed_mod});