Insert cast to enum underlying type when casting to enum type in enum initializer

[reflectronic]

In C#, generally, casts to enum types inside of enum initializers cause things to stop working. For example:

public enum E1
{
    F = 3
}

public enum E2
{
    M = E1.F + E1.F, // Compiles fine
    E = (E1) E1.F + E1.F // Does not work!
}

Removing all casts to enum types when inside an enum initializer avoids these issues.