C++: Remove redundants casts from IR

[MathiasVP]

Consider the following snippet:

struct S {
  void foo() const;
};

void test() {
  S s;
  s.foo();
}

Because foo is a const member function there will be a GlvalueConversion which converts the qualifier from a glval<S> to a glval<const S> before it's passed to foo. This conversion will generate a Convert instruction.

However, there is very little value in keeping those conversions. In fact, it sometimes makes analysis harder for us. For example, the value number of the qualifier to foo (i.e., s after the conversion) is different from any other uses of s since those uses likely have type glval<S>. This fact bit me today!

This PR removes these conversions which previously generated a Convert instruction, but only modified the specifiers (i.e., constness, volatileness, etc.)