strengthen assignment type consistency

The previous check for type consistency of assignments uses base_type_eq,
which permits the expansion of tag types.  This expansion makes dealing with
such types prone to error and costs memory and performance.

Author: Daniel Kroening

src/util/std_code.h

@@ -303,7 +303,7 @@ class code_assignt:public codet
 
     DATA_CHECK(
       vm,
-      base_type_eq(code.op0().type(), code.op1().type(), ns),
+      code.op0().type() == code.op1().type(),
       "lhs and rhs of assignment must have same type");
   }