fix Issue 12581 - [ICE](statement.c, line 713) with invalid assignment + alias this

Make AssignExp::semantic reentrant for `op_overload` call.

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If an expression supports operator overloading, its `semantic` function should not store error expressions in itself until the point of `op_olverload` call (from [a] to [b]).

Example:

Expression *XXXExp::semantic(Scope sc)
{
  // [a]
    ...
    e1 = e1->semantic(sc);  // Bad: if semantic analysis returns ErrorExp,
                            // it is stored in this->e1 immediately.
    if (e1->op == TOKerror)
        return e1;

  // [b]
    Expression *e = op_overload(sc);
    if (e)
        return e;
    ...
}

If `XXXExp::semantic` is called from the `trySemantic` call for `alias this` analysis (from `op_overload` function in `opover.c`), the stored ErrorExp would accidentally escape from the gagged period.