Add TODO in invariant_set seeking to understand magic number

When trying to eliminate hard-coded uses of "8" in the code base I came
across this piece of code, which I don't actually understand the purpose
of. Should someone decipher invariant_set.cpp at some later date:
perhaps they can clean this up.

Author: tautschnig

src/analyses/invariant_set.cpp

@@ -364,6 +364,8 @@ void invariant_sett::add_type_bounds(const exprt &expr, const typet &type)
   {
     std::size_t op_width=to_unsignedbv_type(type).get_width();
 
+    // TODO - 8 appears to be a magic number here -- or perhaps this should say
+    // ">=" instead, and is meant to restrict types larger than a single byte?
     if(op_width<=8)
     {
       unsigned a;