Byte operator lowering: additional preconditions in instantiate_byte_array

No code changes, just additional safety checks to guard against
implementation errors: instantiate_byte_array requires a byte array or
vector typed source operand. Other kinds of arrays/vectors are not
supported or expected.

Author: tautschnig

src/solvers/lowering/byte_operators.cpp

@@ -405,6 +405,10 @@ static exprt::operandst instantiate_byte_array(
       src.operands().begin() + narrow_cast<std::ptrdiff_t>(upper_bound)};
   }
 
+  PRECONDITION(src.type().id() == ID_array || src.type().id() == ID_vector);
+  PRECONDITION(
+    can_cast_type<bitvector_typet>(src.type().subtype()) &&
+    to_bitvector_type(src.type().subtype()).get_width() == 8);
   exprt::operandst bytes;
   bytes.reserve(upper_bound - lower_bound);
   for(std::size_t i = lower_bound; i < upper_bound; ++i)