Move class __cmp lowering to opover.d

[dkorpel]

The goal is to make op_overload return the final expression so it can recursively be called for alias this, getting rid of this special case:

    Expression result;
    if (be.op == EXP.concatenateAssign)
        result = be.op_overload(sc);
    else
        result = be.trySemantic(sc);

See: #8846

In my opinion, the trySemantic check in op_overload should be replaced entirely with a recursive call to op_overload. I tried this, but unfortunately all other BinAssignExps rely on the fact that alias this is tried in op_overload. This should be fixed in a subsequent PR.

It also gets rid of the awkard EXP* pop = null parameter of op_overload.

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