Use constant propagation to dereference left-hand sides

We still fall back to setting values to top if dereferencing fails.

Author: tautschnig

regression/goto-analyzer/constant_propagation_19/main.c

@@ -0,0 +1,10 @@
+#include <assert.h>
+
+int main()
+{
+  int x;
+  int *p = &x;
+  *p = 42;
+  assert(x == 42);
+  return 0;
+}

regression/goto-analyzer/constant_propagation_19/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+--constants --verify
+^EXIT=0$
+^SIGNAL=0$
+^\[main.assertion.1\] file main.c line 8 function main, assertion x == 42: SUCCESS$
+--
+^warning: ignoring

src/analyses/constant_propagator.cpp

@@ -38,12 +38,18 @@ void constant_propagator_domaint::assign_rec(
 {
   if(lhs.id() == ID_dereference)
   {
-    const bool have_dirty = (cp != nullptr);
+    exprt eval_lhs = lhs;
+    if(partial_evaluate(dest_values, eval_lhs, ns))
+    {
+      const bool have_dirty = (cp != nullptr);
 
-    if(have_dirty)
-      dest_values.set_dirty_to_top(cp->dirty, ns);
+      if(have_dirty)
+        dest_values.set_dirty_to_top(cp->dirty, ns);
+      else
+        dest_values.set_to_top();
+    }
     else
-      dest_values.set_to_top();
+      assign_rec(dest_values, eval_lhs, rhs, ns, cp);
   }
   else if(lhs.id() == ID_index)
   {