Fetch for read in nested property assignments

Fetch for read in nested property assignments

    $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).

Closes GH-5250.