Fetch for read in nested property assignments

[iluuu1994]

$a->b->c = 'd';

is now compiled the same way as

$b = $a->b;
$b->c = 'd';

That is, we perform a read fetch on $a->b, rather than a write fetch.

This is possible, because PHP 8 removed auto-vivification support for objects, so $a->b->c = 'd' may no longer modify $a->b proper (i.e. not counting interior mutability of the object).

[iluuu1994]

The fact that the lhs of -> is fetched using W/RW in these given contexts was actually really useful for data classes (#13800). So, I no longer want to make this change.