Eliminate overhead of fetching and testing `NULL` attributes in `STORE_ATTR` specializations for new objects

Consider

class C:
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c
C(1,2,3)

This produces a trace looking something like this:

...
_CHECK_AND_ALLOCATE_OBJECT
_CREATE_INIT_FRAME
_PUSH_FRAME
# Some guards
_LOAD_FAST_BORROW_1
_LOAD_FAST_BORROW_0
# Some more guards
_STORE_ATTR_INSTANCE_VALUE
# Some more guards
_LOAD_FAST_BORROW_2
_LOAD_FAST_BORROW_0
# Some more guards
_STORE_ATTR_INSTANCE_VALUE
# Some more guards
_LOAD_FAST_BORROW_3
_LOAD_FAST_BORROW_0
# Some more guards
_STORE_ATTR_INSTANCE_VALUE
...

Each of those _STORE_ATTR_INSTANCE_VALUE reads the old value out of memory and then conditionally decrefs it.
But in this case we know that the old value was NULL so we can just overwrite it.
So we can replace this:

        PyObject **value_ptr = (PyObject**)(((char *)owner_o) + offset);
        PyObject *old_value = *value_ptr;
        FT_ATOMIC_STORE_PTR_RELEASE(*value_ptr, PyStackRef_AsPyObjectSteal(value));
        if (old_value == NULL) {
            PyDictValues *values = _PyObject_InlineValues(owner_o);
            Py_ssize_t index = value_ptr - values->values;
            _PyDictValues_AddToInsertionOrder(values, index);
        }
        Py_XDECREF(old_value);

with this:

        PyObject **value_ptr = (PyObject**)(((char *)owner_o) + offset);
        FT_ATOMIC_STORE_PTR_RELEASE(*value_ptr, PyStackRef_AsPyObjectSteal(value));
        PyDictValues *values = _PyObject_InlineValues(owner_o);
        Py_ssize_t index = value_ptr - values->values;
       _PyDictValues_AddToInsertionOrder(values, index);

On Aarch64, this reduces the number of machine instructions from 48 to 26.

The same reasoning also applies to _STORE_ATTR_SLOT where it reduces the number of machine instructions from 32 to 14.

See also https://github.com/python/cpython/issues/134584

We can probably remove some of those guards as well, but that's a separate issue.

Commencez par localiser les spécialisations de l’interpréteur CPython nommées STORE_ATTR_INSTANCE_VALUE et STORE_ATTR_SLOT, puis lisez l’issue associée 134584 pour le contexte. Comparez leur gestion des objets nouvellement alloués avec les chemins proposés et vérifiez que la modification préserve la gestion de l’ordre d’insertion tout en éliminant les lectures de NULL, les decrefs et la surcharge d’instructions signalée.