C++: Simplify the definition of SemExpr for range analysis

[MathiasVP]

In C/C++ there are many implicit conversions which must be handled as part of range analysis. For example, in the following:

void test(short s) {
  if(s < 5) { ... }
}

there's a short-to-int conversion on s. Previously, we made the new range analysis library handle this by wrapping the definition of a C/C++ SemExpr inside an EquivalenceClass so that the expression (int)s and the expression s was identical.

However, @aschackmull made me aware that the range analysis library already has support for "this expression reads this variable" to handle stuff like +n < 5 in Java. So we can get equivalent functionality by simply making these "safe conversions" copy-value expressions, and the range analysis library will ensure that a bound on (int)s in the example above is correctly identified as a bound on s.

cc @aschackmull what do you think of this? I know there are other changes you'd like to see to make your library sharing easier, but I think this is a good first step 😅.

[MathiasVP]

DCA was uneventful 🎉

[aschackmull]

If you are happy with the test changes, then I'd say the code changes LGTM!