Support typecast from bv_typet to pointer_typet

The fixes in 848e633b67 result in type casts from bv_typet to various
other bitvector types being generated. Most cases were addressed in that
commit, but support for casting to pointers was omitted. This follow-up
commit fixes this, and adds a test specifically covering this case.

regression/cbmc/Typecast3/main.c

@@ -0,0 +1,15 @@
+#include <assert.h>
+#include <stdlib.h>
+
+int main()
+{
+  int **s = malloc(sizeof(int *));
+  for(int i = 0; i < sizeof(int *); ++i)
+  {
+    // A correct program would use (char *)s, but updating the pointer allows us
+    // to reproduce a bug in CBMC (missing support for typecasts from bv_typet
+    // to pointers).
+    *((char *)&s + i) = 0;
+  }
+  assert(s[0] == 0);
+}

regression/cbmc/Typecast3/test.desc

@@ -0,0 +1,21 @@
+CORE
+main.c
+
+^EXIT=10$
+^SIGNAL=0$
+^VERIFICATION FAILED$
+--
+^warning: ignoring
+--
+This test makes sure we support bv_typet -> pointer type casts, avoiding
+warnings like:
+
+warning: ignoring typecast
+  * type: pointer
+      * width: 64
+      0: signedbv
+          * width: 32
+          * #c_type: signed_int
+  0: typecast
+      * type: bv
+          * width: 64

src/solvers/flattening/bv_pointers.cpp

@@ -253,11 +253,10 @@ bvt bv_pointerst::convert_pointer_type(const exprt &expr)
 
     if(op_type.id()==ID_pointer)
       return convert_bv(op);
-    else if(op_type.id()==ID_signedbv ||
-            op_type.id()==ID_unsignedbv ||
-            op_type.id()==ID_bool ||
-            op_type.id()==ID_c_enum ||
-            op_type.id()==ID_c_enum_tag)
+    else if(
+      op_type.id() == ID_signedbv || op_type.id() == ID_unsignedbv ||
+      op_type.id() == ID_bool || op_type.id() == ID_c_enum ||
+      op_type.id() == ID_c_enum_tag || op_type.id() == ID_bv)
     {
       // Cast from integer to pointer.
       // We just do a zero extension.