Remove vectors: Boolean expressions do not require signed bitvectors

Any bitvector type will do as all we need to produce is a constant with
all bits set (see
https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html: "Vectors are
compared element-wise producing 0 when comparison is false and -1
(constant of the appropriate type where all bits are set) otherwise.").

Author: tautschnig

src/goto-programs/remove_vector.cpp

@@ -188,7 +188,6 @@ static void remove_vector(exprt &expr)
       const auto dimension = numeric_cast_v<std::size_t>(vector_type.size());
 
       const typet &subtype = vector_type.element_type();
-      PRECONDITION(subtype.id() == ID_signedbv);
       exprt minus_one = from_integer(-1, subtype);
       exprt zero = from_integer(0, subtype);