GH-141890: Fix GC double-counting of marked objects

[brandtbucher]

This move happens twice: once here, and once in gc_collect_increment (the caller).

• Issue: Incremental GC double-counts work done during the marking phase #141890

[sergey-miryanov]

JFYI, #127519

With marking added to the cyclic GC (#127110) we spend a lot of the time in the GC forming transitive closures, both for marking and for the increments of the incremental GC.

Unfortunately the current algorithm has a couple of mistakes in it. One harmful, one beneficial.

The beneficial one is counting the initial mark twice. This helps because it reduces the cost of GC on heaps with little or no garbage
The harmful one is allowing the amount of work done to grow in proportion to the heap size.

Unfortunately, #127519 was reverted.

I had planned to try to fix it, but it's still on my to-do list.

[brandtbucher]

Ah, interesting. Let's benchmark this change. If the current version is better, we can just multiply by two to be explicit instead of the current code (which looks like a mistake, because originally it was). :)