Adjust the value in objects' insertion order array to the index - position.

[markshannon]

Currently, we store an array of bytes at the end of the inline value array of objects to record the insertion order.
This is expensive as we need to compute not only the value to insert, but where to insert it. Here's the code:

     PyDictValues *values = _PyObject_InlineValues(owner_o);
     Py_ssize_t index = value_ptr - values->values;
     _PyDictValues_AddToInsertionOrder(values, index);

_PyObject_InlineValues requires some lookup, as it depends on tp->basicsize

    return (PyDictValues *)((char *)obj + tp->tp_basicsize);

but it is _PyDictValues_AddToInsertionOrder that is the slowest:

    int size = values->size;
    uint8_t *array = (uint8_t *)&values->values[values->capacity];
    array[size] = (uint8_t)ix;
    values->size = size+1;

If instead of storing the delta of the index and position, instead of just the index, for the majority of objects the insert order array will go from { 0, 1, 2, 3, 4, ... } to { 0, 0, 0, 0, 0, ... }
And if the delta is zero, we don't need to store anything.

     PyDictValues *values = _PyObject_InlineValues(owner_o);
     Py_ssize_t index = value_ptr - values->values;
     Py_ssize_t delta = index - values->size;
     if (delta != 0) {
        /* This is the expensive part */
        _PyDictValues_AddToInsertionOrder(values, delta);
    }
    values->size++;

In the JIT we can track the size of the inline values, and know when delta will be zero. Reducing the above code to

    values->size = KNOWN_SIZE;