Handle bool expressions in implicit_typecast_arithmetic

We use Booleans instead of int as type of binary predicates. This must
not preclude mixing binary predicates with bitvector-typed expressions.

Fixes: diffblue#5103

Author: tautschnig

regression/ansi-c/int128_and_bool1/main.c

@@ -0,0 +1,6 @@
+int main(int argc, char *argv[])
+{
+  unsigned __int128 z;
+  z = (unsigned __int128)argc;
+  z += (argc < 1);
+}

regression/ansi-c/int128_and_bool1/test.desc

@@ -0,0 +1,7 @@
+CORE gcc-only
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^CONVERSION ERROR$

src/ansi-c/c_typecast.cpp

@@ -627,8 +627,10 @@ void c_typecastt::implicit_typecast_arithmetic(
   if(max_type==LARGE_SIGNED_INT || max_type==LARGE_UNSIGNED_INT)
   {
     // get the biggest width of both
-    std::size_t width1 = to_bitvector_type(type1).get_width();
-    std::size_t width2 = to_bitvector_type(type2).get_width();
+    std::size_t width1 =
+      type1.id() == ID_bool ? 1 : to_bitvector_type(type1).get_width();
+    std::size_t width2 =
+      type2.id() == ID_bool ? 1 : to_bitvector_type(type2).get_width();
 
     // produce type
     typet result_type;